Executive Summary

Threats from seasonal and pandemic influenza persist in a world still battling the COVID-19 pandemic. As another highly contagious respiratory virus, influenza causes a spectrum of disease, ranging from mild to fatal. Globally, it causes up to 1 billion infections annually and kills an estimated 290,000 to 650,000 people each year. The regular appearance of seasonal influenza outbreaks year after year can dull public concern for the significant amount of severe illness and death caused by the virus, particularly among the elderly and young children, and for those living in low- and middle income countries (LMICs).

The chronic lack of urgency to address such a heavy burden may be compounded by the apparent absence of seasonal influenza during the COVID-19 pandemic. Experts also worry that the COVID-19 crisis is obscuring the very real threat of a pandemic caused by another respiratory disease, such as influenza, and that we are missing out on the opportunity to prepare for a future crisis that could be at least as deadly and disruptive as COVID-19, if not more so. Public health experts have estimated, for example, that the influenza pandemic of 1918-19 killed more than 50 million people globally.

The IVR addresses the significant barriers and opportunities for preventing influenza and lays out a concise 10-year plan to leverage the lessons from the COVID-19 pandemic and reduce the threat of influenza globally through improving the production and effectiveness of strain-specific seasonal influenza vaccines and by advancing the development, licensure, and production of universal influenza vaccines. 

The IVR provides a focused blueprint for action that will allow the world to apply the lessons learned from COVID-19 vaccine development as a springboard for launching a new era for influenza vaccine R&D. 

Read the Full Executive Summary

The COVID-19 pandemic has dramatically illustrated the catastrophic health, social, and economic consequences that a severe pandemic can generate in the 21st century. It also has demonstrated how rapidly viral respiratory pathogens can traverse the planet, leaving little opportunity for just-in-time preparedness. Influenza viruses periodically cause severe pandemics through antigenic shift and generation of novel reassortant viruses, and we don’t know when the next one will occur or how severe it will be. While public health officials and others have created a strong foundation for pandemic preparedness, additional efforts are still required.

Current Influenza Vaccines are Suboptimal 

Vaccines are essential for protecting populations from seasonal and pandemic influenza, yet our current influenza vaccines and vaccination programs fall short. First, antigenic mismatches can occur between circulating viruses and viruses used for generating strain-specific vaccines, and the vaccines need to be reformulated and readministered annually. Second, even in years with good antigenic matches, vaccine effectiveness is often suboptimal, particularly in the elderly. Third, researchers have yet to unlock the keys to developing influenza vaccines that generate durable immunity—for example, lasting 5 to 10 years. Fourth, researchers also have yet to develop broadly protective vaccines that can protect against multiple strains of influenza, including novel pandemic viruses. Finally, although seasonal influenza vaccines are widely available, many countries—particularly low- and middle-income countries (LMICs)—do not have robust seasonal influenza vaccination programs in place, and vaccine uptake varies across nations and populations. Universal and durable vaccines that protect against all current and future strains of influenza would be a tremendous breakthrough by ensuring that vaccines are readily available in sufficient quantities at the onset of future influenza pandemics.

The Influenza Vaccines R&D Roadmap 

The Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota, with support from the Wellcome Trust, has created this globally oriented influenza vaccines research and development (R&D) roadmap (IVR). The IVR is intended to serve as a strategic planning tool to facilitate R&D, coordinate funding, and promote stakeholder engagement in R&D aimed at improving seasonal influenza vaccines and generating new broadly protective or universal influenza vaccines. The impetus for this project came from priorities published in 2019 by the Global Funders Consortium for Universal Influenza Vaccine Development (Bresee 2019). Key components of the IVR include determining critical issues and challenges for improving vaccines, prioritizing research activities, and identifying a set of realistic goals and aligned milestones in key topic areas. The IVR takes into account the potential for different needs regarding influenza vaccine characteristics in different populations, economies, and geographic regions. Primary audiences include scientific and clinical researchers, funders, public health policymakers, industry scientists and business leaders, regulators, and communications/advocacy specialists. Additional audiences may extend to a wider group of global health leaders, policymakers, and government officials. 

The IVR development process has engaged a wide range of stakeholders across scientific disciplines, public and private sectors, and international communities to build consensus on R&D priorities and identify strategies for addressing them. The process has included identifying and reviewing other relevant strategic plans from high-level organizations such as the World Health Organization (WHO) and government entities, discussing scientific challenges and knowledge gaps with a range of subject-matter experts (SMEs), and conducting in-depth reviews of draft roadmap documents (including a public comment period for written feedback). After publication and launch of the roadmap, efforts will focus on dissemination (through targeted communication strategies), implementation, and monitoring, evaluation, and adjustment (ME&A) over time. 

Advising CIDRAP’s core team throughout the project is a small steering group of senior leaders from the WHO, the Wellcome Trust, the Sabin Vaccine Institute, the Bill & Melinda Gates Foundation, and the Task Force for Global Health. In early 2019, the steering group advised CIDRAP on the establishment of a global IVR development taskforce of SMEs, who offer a wide range of knowledge and experience in influenza vaccines R&D.

In September and October 2020, CIDRAP held a series of online meetings to engage a larger group of SMEs, including experts from academia, industry, government, and nongovernmental organizations, to discuss the IVR initiative and provide additional feedback on the draft roadmap. In January and February 2021, CIDRAP posted the revised roadmap online for public comment, receiving critical feedback from over 100 individuals in 26 countries representing 88 different organizations in academia, government, industry, and nongovernmental organizations. The roadmap was revised further to incorporate feedback from the public comment period. 

Implementation of the roadmap will require working closely with partner organizations to determine how the IVR strategic goals and milestones can be achieved. Key steps will include identifying funding sources, establishing ownership for certain activities in the roadmap, and obtaining commitments from organizations to move roadmap activities forward. The IVR will be updated periodically as new information becomes available, goals and milestones are achieved, and new challenges occur. The IVR taskforce will play a key role in guiding implementation of the IVR and determining the need for additional project-management tools, potentially to include detailed implementation plans, an online dashboard of progress, tracking of influenza vaccine R&D funding, and updated summaries of influenza vaccine R&D priorities over time.

Influenza Disease Burden 

Seasonal influenza epidemics, caused by influenza A and B viruses, account for an estimated 3 million to 5 million cases of severe illness and 290,000 to 650,000 respiratory deaths each year worldwide (WHO 2019) and result in substantial economic burdens (Sah 2019). Disproportionately higher rates of illness and death from influenza occur in low-income countries in sub-Saharan Africa and Southeast Asia, particularly among the elderly and children under 5 years (Bartoszko 2021, Iuliano 2018, Wang 2020). Influenza viruses are maintained in human populations by person-to-person transmission of antigenically diverse viruses that circulate seasonally in the Northern and Southern Hemispheres and year-round in tropical regions (Young 2020). Influenza A viruses from animal reservoirs (mainly wild birds and swine) to which humans have limited immunity can spill over to people, creating the ongoing threat of pandemic influenza. Novel influenza A viruses with increased pathogenicity and transmissibility have the potential to cause severe pandemics, as occurred with the 1918 pandemic, which is estimated to have caused over 50 million deaths (Morens 2019). Less severe influenza A pandemics occurred in 1957, 1968, and 2009. Influenza B viruses account for about 25% of seasonal influenza infections, with wide variation from year to year (Hensen 2020); no pandemics have been attributed to influenza B. 

Influenza Vaccines 

Vaccination remains the cornerstone of reducing the burden of seasonal influenza and enhancing pandemic preparedness (WHO 2020, WHO 2019). While current influenza vaccines reduce infection rates and influenza-related complications and deaths (Chung 2020, Paules 2019a), their overall effectiveness is suboptimal. From 2004 to 2018, average US estimates of influenza vaccine effectiveness against medically attended illness ranged from 10% to 60% (CDC 2020). During the 2016-17 influenza season, the overall vaccine effectiveness at six international sites (Valencia, Canada, St. Petersburg, Mexico, Moscow, and Turkey) was estimated at 27% (Baselga-Moreno 2019). 

“There is an urgent need for better tools to prevent, detect, control, and treat influenza, including more effective vaccines . . . that would instill public confidence and uptake, especially in low- and middle-income countries.” 

- WHO Global Influenza Strategy 2019 to 2030

Although the vaccines are reformulated biannually to align with predicted circulating influenza viruses in the Northern and Southern Hemispheres, frequent genetic changes result in different proportions of virus subtypes/lineages circulating. The ability to accurately predict predominant circulating strains is also limited. Therefore, at times vaccine strains may be antigenically distinct from circulating viruses, creating mismatches. Vaccine protection against influenza may still be suboptimal even when the antigenic match appears good, indicating that other aspects may also be involved in determining vaccine efficacy, such as pre-existing partial immunity or the influence of various other host factors on the immune response (Dhakal 2019, Allen 2018, Harding 2018, Lewnard 2018, McElhaney 2020b, Zost 2017). In addition, the duration of protection is notably short, requiring vaccines to be readministered annually, which poses challenges for immunization programs, particularly in LMICs. Recognizing the limitations of current influenza vaccines, the WHO’s comprehensive Global Influenza Strategy 2019-2030 highlights the urgent need for more effective influenza vaccines to prevent and control seasonal influenza and to enhance pandemic preparedness (WHO 2019). 

Several types of influenza strain-specific vaccines are licensed and in widespread use, including split or subunit inactivated vaccines, recombinant vaccines (both administered by intramuscular injection), and live-attenuated influenza vaccines, which are administered intranasally (Gouma 2020a, Mathew 2020). Although some influenza vaccines are produced via cell-culture methods (Rajaram 2020), most of the estimated 1.48 billion doses of seasonal vaccine produced each year are manufactured using conventional egg-based production methods (Estrada 2019, Sparrow 2021). Production of egg-based and recombinant cell-culture influenza vaccines require at least 6 months to complete (Paules 2019a), limiting their usefulness in responding rapidly to the emergence of a pandemic virus. 

Because they continually evolve, influenza viruses are often referred to as “moving targets,” given the hypervariable antigenic sites on the virus’s hemagglutinin (HA) and neuraminidase (NA) surface proteins (Francis 2019, McLean 2020, Paules 2019b). These sites have traditionally served as the primary antigens in strain-specific seasonal vaccines. Recent efforts to improve the immunogenicity of these vaccines have focused on the use of high-dose formulations, alternative vaccine delivery methods, and the addition of adjuvants, which can be added to boost the immune response (Wei 2020). Achieving significant reductions in morbidity and mortality worldwide, however—particularly among older adults and other at-risk populations—will require novel approaches (Madsen 2020, McElhaney 2020a). 

Several policy initiatives and research funding programs have been launched to stimulate influenza vaccine research and highlight the need for transformative technologies. Examples include the Collaborative Influenza Vaccine Innovation Centers (CIVICs) program from the US National Institute of Allergy and Infectious Diseases (NIAID) (NIAID 2020), the EU-India Joint Call (EEAS 2018), and the Bill & Melinda Gates Foundation Grand Challenge for Universal Influenza Vaccine Development (BMGF 2018a). The ultimate goal of these efforts is to develop durable, broadly protective vaccines suitable for widespread use. Innovations being considered include new vaccine approaches (such as messenger ribonucleic acid [mRNA] lipid nanoparticles and recombinant protein nanoparticles) and novel constructs that target conserved viral epitopes to induce broadly neutralizing antibodies (Wu 2020) and cross-reactive T cell responses (Wei 2020). 

Influenza Vaccine Definitions 

There is no commonly accepted definition of “universal” influenza vaccines, which is often used interchangeably with “broadly protective” or “next-generation” vaccines. Krammer and colleagues (Krammer 2018a) suggest that “universal” refers to “lifelong protection against all drift and shift variants, including seasonal influenza A and B, pandemic, and zoonotic strains,” whereas “broadly protective” refers to protection against a subset of influenza viruses (such as all current seasonal strains and their drift variants or against all influenza A viruses) for at least several years. The NIAID strategic plan states that the ultimate goal is to develop universal vaccines that are at least 75% effective against symptomatic influenza infection, protect against phylogenetic groups 1 and 2 influenza A viruses, provide durable protection for at least 1 year, and are suitable for all age-groups (Erbelding 2018). 

“Next-generation influenza vaccines” can also be defined according to their various potential roles, such as: high-performance seasonal vaccines (improved existing vaccines); supraseasonal vaccines (seasonal vaccines with consistent annual efficacy without reformulation over two or more seasons); vaccines for pandemic preparedness (stockpiled for immediate use during a pandemic); vaccines for pandemic response (rapid production at the onset of a pandemic); and universal vaccines (consistent efficacy against all influenza A and B viruses, including drifted seasonal and pandemic viruses, ideally for at least 3 years, achieving at least 75% reduction in medically attended lower respiratory tract disease or hospitalization) (Kanekiyo 2020). 

In a research context, the term “universal influenza vaccine” serves as an aspirational concept that ideally encompasses four attributes: (1) breadth of protection against diverse influenza viruses (including influenza A and B viruses); (2) robust immunogenicity and safety in different age-groups and special populations (e.g., immunocompromised individuals, pregnant women); (3) durability (at least several years’ protection against seasonal influenza viruses); and (4) suitability for global use. Outcomes of the research may include one or more influenza vaccine products for different purposes, markets, or populations, each with different characteristics and target product profiles. If durable, universal influenza vaccines can be developed that are suitable for use in LMICs, this will have a dramatic impact on the entire influenza vaccination enterprise. 

This roadmap uses the following definitions regarding influenza vaccine R&D: 

• Universal influenza vaccine: Offers protection against all influenza A and B viruses, including seasonal viruses and existing or emergent zoonotic viruses with pandemic potential. 
• Broadly protective influenza vaccine: Offers protection against multiple influenza viruses but does not meet the criteria for a universal vaccine. For example, a broadly protective vaccine could confer protection against all strains within a single HA subtype (subtype-specific), multiple HA subtypes within a single group (multi-subtype), all group 1 or group 2 influenza A viruses (pan-group), or all influenza B viruses. 
• Next-generation influenza vaccine: Involves a different strategy than currently licensed seasonal vaccines to elicit protective immune responses against influenza viruses (e.g., uses different vaccine platforms or targets antigens other than, or in addition to, the variable HA head epitopes), demonstrating an improvement over current vaccines in durability, efficacy, or breadth of protection. Both universal vaccines and broadly protective vaccines could be considered next-generation vaccines, but next-generation vaccines also could include strain-specific vaccines if they offer significant public health advantages, such as greater durability or a 15% to 20% increase in effectiveness. 

Examples of how other groups have defined the above terms can be found in Appendix 2. 

 Roadmap Scope and Structure

Recent efforts to develop R&D roadmaps in other fields, such as medical countermeasure development for WHO priority diseases (WHO R&D Blueprint Initiative), have informed the structure of the IVR. Each IVR section contains an overview of key issues, barriers, and knowledge gaps. High-level strategic goals within these topics are identified, followed by associated actions (milestones) required to achieve them. The milestones include target dates for completion and reflect SMART (Specific, Measureable, Achievable, Realistic/Relevant, and Time-sensitive) criteria, to the degree feasible. High-priority milestones, listed in the text and in Appendix 3, are those that are on the critical path to improving seasonal influenza vaccines or to generating universal influenza vaccines or are considered essential to advance influenza vaccine R&D, based on consensus agreement among IVR taskforce members. 

The IVR is organized into six topic areas:

In addition to influenza vaccine development, other interventions are critical for effective influenza prevention and control. These include the development of influenza vaccines for use in domestic animals, the promotion of technology-transfer for vaccine production in LMICs, and the development and delivery of effective antiviral drugs, monoclonal antibody therapies, and vaccines to prevent other lower respiratory tract infections. Increased availability of influenza vaccines, especially in LMICs, enhanced uptake among high-risk or underserved populations, and broader acceptance of influenza vaccines are also critical ongoing activities, but all these issues are beyond the scope of this roadmap. 

IVR Development and Outcomes

Issue: Continuous evolution of influenza viruses (genetic changes that confer antigenic drift) 

Barriers 

• Antigenic drift results in influenza virus variants that are no longer neutralized by immune responses generated through prior natural infection or strain-specific vaccinations, placing people at continuous risk of infection (Bouvier 2018). 
• The antigenic variability of influenza viruses necessitates frequent changes in vaccine seed strains and limits the potential for seasonal vaccines to generate long-term humoral immunity (Estrada 2019). 
• Antigenic drift can contribute to mismatches between seasonal influenza vaccines and the predominant circulating viruses (Gouma 2020a). 
• Improved understanding of changes in circulating subtypes of influenza virus has driven the need from bivalent A/B, to trivalent A/B, to now quadrivalent A/B vaccines, which demonstrates the complexity of providing vaccine coverage as strains change over time. 

Gaps 

• Although the WHO Global Influenza Surveillance and Response System (GISRS) and the Global Initiative on Sharing Avian Influenza Data (GISAID) have successfully fostered international sharing of influenza virus isolates and gene sequences for years, global surveillance for influenza is not uniformly distributed, with limited data for large populations, including global hot spots for influenza virus diversity. Expanding the geographic range of influenza sequence data, with increased metadata collection and virus sample sharing for phenotypic analysis, could provide important information about global virus evolution, including the impact of travel and host genetics. Additional sequence data could also provide critical early information on an emerging pandemic virus. For example, the first SARS-CoV-2 genetic sequences were made available on GISAID’s EpiCov platform on January 10, 2020, which allowed manufacturers to begin the COVID-19 vaccine development process (Polack 2020). 
• Improved computational methods, including shared databases and modeling approaches, could facilitate better understating of the emergence of viral variants, the relative capacity of acquired viral receptor mutations to interact with the host cells, and the possibility to infer effectiveness of vaccines via simulation. 
• Efforts by the WHO Collaborating Centers and others to improve methods for antigenic characterization of H3N2 viruses have been underway for many years. Progress has been made (including development of moderate throughput plaque reduction assays and the high content imaging-based neutralization test [HINT]), although further improvements are needed for ongoing antigenic characterization of H3N2 viruses to enhance understanding of the public health implications regarding co-circulation of different antigenic variants of this viral subtype (Allen 2018, Harding 2018, Zost 2017). 
• Additional research is needed to determine:
  • How viral changes (e.g., changes in HA-head glycosylation, NA modification, interior protein changes) affect vaccine efficacy and immunogenicity (Allen 2018, Medina 2013, Wille 2020). 
  • How antigenic drift might help predict the impact of selective pressure on conserved epitopes targeted by vaccines (Saad-Roy 2019). 
  • How best to predict the direction and speed of antigenic drift for influenza viruses to improve forecasting ability (Ali 2021, Dadonaite 2019). 
  • The role of new tools (such as computational analysis and systems biology) to facilitate understanding of influenza virus evolution (Erbelding 2018, Viboud 2020). 
  • Reasons for antigenic mismatches between vaccine strains and circulating strains and how they could be avoided. 

Issue: Emergence of novel influenza viruses with pandemic potential (antigenic shift) 

Barriers 

• Within the range of different HA and NA influenza A virus subtypes found in nature, only three HAs (H1, H2, H3) and two NAs (N1, N2) have been identified in pandemic viruses. It is possible, however, that influenza A viruses of other HA and NA subtypes could generate a pandemic strain through antigenic shift, and this uncertainty is an important barrier to predicting the emergence of pandemic viruses (Morens 2019). 
• Numerous influenza A viruses are enzootic in animal reservoirs (e.g., wild and domestic birds, swine, horses, dogs), and humans are largely immunologically naive to these viruses. Some have the potential to cross the species barrier and infect humans following exposure at the human-animal interface (Long 2019). The inability to predict which strains will spill over to immunologically naive populations is an important challenge in understanding the risks. 
• Novel or unanticipated viral strains are not targeted by current seasonal vaccine technology. Although strain-specific vaccines are being produced for certain H5 and H7 viruses, vaccines against other subtypes with pandemic potential need to be addressed. For example, previously endemic (1957 to 1968) human H2N2 viruses could re-emerge, or novel avian influenza A viruses could spill over into humans and become more easily transmissible from person to person (Reneer 2019, Trucchi 2019). 

Gaps 

• Important needs include:
  • Renewed commitments for ongoing influenza virus surveillance programs in wild and domestic animals living near people, particularly for swine and poultry (Erbelding 2018, Neumann 2019). 
  • Improved surveillance and rapid characterization to identify and monitor emergent influenza viruses for changes in person-to-person transmissibility and increased virulence (Neumann 2019). 
  • Additional information to clarify how and where influenza viruses cross species barriers (Schrauwen 2014). 
  • More research to (1) identify mutations in the viral genome that confer binding to human-type receptors (Neumann 2019), (2) identify the sialic acid receptors present in human airways, (3) assess which of those are preferential for influenza viruses, (4) identify amino acid changes in animal HAs that confer efficient binding to these preferred receptors (Neumann 2019), and (5) better understand the phenotypes that promote cross-species transmission and emergence of zoonotic viruses. 

Issue: Influenza virus transmissibility and spread, particularly with regard to population dynamics and the impact of vaccination 

Barriers 

• Influenza virus transmission has been historically difficult to study, and the impact of vaccination on interrupting transmission in humans is unknown (Erbelding 2018, Leung 2021). 
• Capacity to conduct surveillance for influenza is limited in some areas of the world, particularly in tropical and subtropical countries (WHO 2016a). 
• Resources to conduct comprehensive influenza virus transmission studies in animals are limited, and further efforts are needed to optimize animal models for studying transmission dynamics (Neumann 2019). (Also see Topic 5: Animal Models and the Controlled Human Influenza Virus Infection Model.)

Gaps 

• Efforts are needed in several other areas:
  • Ongoing assessment of transmissibility of mutant and reassortant viruses to predict the pandemic potential of novel influenza viruses (Neumann 2019). 
  • Enhanced understanding of factors associated with transmissibility to allow better modeling of population dynamics and mechanisms to interrupt viral transmission cycles (Crank 2019, Erbelding 2018). 
  • Better definition of the level and type of herd immunity required in a given human population to prevent viral transmission (Erbelding 2018). 
  • Clarification of the impact of vaccination on spread of influenza. 
  • Further study of the role of viral interference (i.e., where one respiratory virus can block infection with another through stimulation of antiviral defenses in the airway mucosa) on transmission dynamics (Anchi 2020). 
  • The inclusion of aerobiologists, engineers, and other experts not generally involved in influenza research to improve transmission knowledge. 

Strategic Goals and Aligned Milestones for Topic 1: Virology Applicable to Vaccine Development

Additional R&D Priorities for Topic 1: Virology Applicable to Vaccine Development 

• Identify mechanisms to maximize the use of existing tools (such as the Influenza Risk Assessment Tool [IRAT] from the US CDC and the Tool for Influenza Pandemic Risk Assessment [TIPRA] from the WHO) for tracking novel influenza viruses and assessing their risk to humans. This may include taking steps to standardize and share risk assessments and risk assessment protocols across the two systems. 
• Increase collaboration with experts in environmental sciences (e.g., aerobiologists, engineers, ventilation experts) for influenza virus transmission research. 
• Encourage collaborations among influenza virologists, surveillance teams, immunologists, and epidemiologic mathematical modelers to generate insights on population dynamics of immunity, transmission dynamics, burden of disease (especially in under-surveilled LMICs), and seasonality of disease under changing climate conditions. 
• Consider expanding the US CDC’s H5N1 Genetic Changes Inventory to include other influenza viruses of concern (e.g., H7N9, H5N6, H5N8, H3N2v) and to capture new mutations. 
• Implement efforts to characterize epitopes on emerging viruses by generating panels of mAbs against the HA and NA glycoproteins (and perhaps other epitopes) as new variants arise. 
• Further study the impact of viral interference on transmission dynamics, which may be important for clinical research into new vaccines (Anchi 2020). 
• Research the impact of vaccination on the spread of influenza in diverse communities. 
• Develop mechanisms to assess influenza viral shedding during infection, such as measuring viral load in expired air over the course of an infection. 
• Enhance understanding of viral and host factors associated with virus transmission. 
• Continue to identify and monitor emergent influenza viruses for changes in human-to-human transmissibility and enhancement of virulence (Neumann 2019). 
• Continue to determine mechanisms by which influenza viruses cross species barriers (Schrauwen 2014). 
• Consider the utility of developing a vaccine for H2N2 and creating a stockpile, as is being done for H5 and H7 strains. 
• Provide in-country training for new participants in influenza surveillance or clinical research, to include regulators, to expedite reviews of protocols. This should also involve agreements regarding host-nation clinical samples leaving the country for centralized testing.

Issue: Fundamental understanding of human immunology relevant to improving influenza vaccines 

Barriers 

• Much of the immunologic research needed for improving influenza vaccines depends on access to clinical samples, which may not be readily available, particularly from commercial entities. 
• Research efforts may be complicated by genetic differences among populations and environmental conditions, such as potential for exposure to other pathogens. 

Gaps 

• To develop improved influenza vaccines, a better understanding of the following is needed regarding:
  • Human immunology (Morens 2019), including the role of host factors in immune responses to vaccines and correlates of protection by immune compartment. 
  • Differences between immune responses to influenza virus infection and influenza vaccination (Clemens 2018, Krammer 2019a, Lopez 2020). 
  • Cooperation across the immune system, including the polyclonal antibody response cooperation—synergistic, additive, or interfering—and immune homeostasis. 
  • Characterization of innate and adaptive immune responses to influenza, including the critical immune factors required for inducing broad, durable protection (Erbelding 2018, Topham 2019) and the mechanisms of vaccine-induced immunity (Cortese 2020). 
  • Mechanisms of non-neutralizing immune effectors and their potential for immune protection, immune interference, or disease enhancement (Bouvier 2018, Crowe 2019, Erbelding 2018). 
  • The processes by which immune dysregulation may contribute to severe influenza (Bouvier 2018, Erbelding 2018). 

Issue: The humoral response to influenza virus infection and vaccination 

Barriers 

• Variable antigens on the HA globular head exhibit immunodominance over the more conserved viral antigens (Krammer 2019a). This creates obstacles for developing new vaccine constructs that are targeted to more conserved domains of the virus (Zost 2019). 
• Manufacturing and price constraints limit the number of antigens that can be included in new vaccine constructs. 

Gaps 

• Further knowledge is needed regarding:
  • The dynamics among B cell subsets following influenza infection and how to influence those dynamics to induce durable memory driven by long-lived plasma cells in the bone marrow (Krammer 2019a). 
  • What level of B cell response to conserved antigens is sufficient to provide protection, either independently or in combination with T cell responses. 
  • The role of NA antibodies in contributing to protection from infection (Eichelberger 2019, Eichelberger 2018, Krammer 2018b, Yamayoshi 2019). (Also see Topic 3: Vaccinology for Seasonal Influenza Vaccines.) 
  • Immunodominance hierarchies in antibody response (Angeletti 2018, Krammer 2019a). 
  • The differences in mechanisms of humoral immunity between protection against infection (sterilizing immunity) and protection from symptomatic or severe disease (Krammer 2019b, Yamayoshi 2019). 
  • The role of stem-specific antibodies in viral fitness and immune escape. 
  • Which neutralizing and non-neutralizing B cell responses enable long-lasting immunologic protection post-infection or vaccination (Coughlan 2018, Krammer 2019a). 

Issue: The cell-mediated response to influenza virus infection and vaccination 

Barriers 

• A robust memory CD8 T cell response to influenza infection can protect against severe disease; however, strong CD8 T cell responses may lead to excessive pro-inflammatory reactions and enhanced immunopathology. This creates a challenge for optimizing vaccine-induced memory CD8 T cell responses without causing adverse clinical outcomes (Souquette 2018). 

Gaps 

• The influenza-specific CD4 T cell repertoire in humans is highly diverse with regard to abundance, specificity, and functionality, making it difficult to define an optimal strategy for vaccine-induced T cell-mediated immunity (Sant 2018, Sant 2019). 
• Improved, more affordable, and standardized T cell assays or markers of T cell function are needed to better assess T cell immunity. 
• More information is need regarding:
  • The T cell response in protecting against severe influenza, both directly and in potentiating the response of other lymphoid cells (Jansen 2019, Sant 2019). 
  • The determinants of beneficial versus detrimental T cell responses. 

Issue: Immune response to influenza virus infection in different anatomic compartments 

Gaps 

• The immune response to influenza viruses may vary by anatomic or cellular compartments (particularly in regard to reassortment/vaccine strain interference) and the types of immune responses that an effective vaccine must elicit in the various compartments are not fully elucidated (Morens 2019). 
• The route of vaccine administration (e.g., intranasal, other mucosal route, intramuscular, transdermal) affects key aspects of the protective immune response in the various immunologic compartments, but feasible-to-use biomarkers of the different responses for influenza protection are not sufficiently established to guide vaccine development strategies (Calzas 2019, Morens 2019). 
• Better understanding of the following is needed:
  • Age-associated changes in lung immune cells in response to influenza virus infection to inform the development of improved vaccines for the elderly (Nguyen 2021). 
  • The role of mucosal immunity in the protective response to influenza infection, including the potential magnitude and scope of mucosal immunity elicited by influenza infection or vaccines and identification of factors affecting the degree of protection following intranasal vaccination or mucosal vaccination by other routes (Calzas 2019, Epstein 2018, Krammer 2019a).

Issue: The role of immune imprinting and prior exposure to influenza infection or vaccine on influencing the immune response to future influenza vaccination and viral infection over time 

Barriers 

• The immune response to influenza may be significantly influenced by early childhood exposure to specific strains of influenza viruses (referred to as immune imprinting) through natural infection or vaccination, leading to different risks of disease from various influenza subtypes (Gostic 2019, Nayak 2019, Valkenburg 2017). The development of universal vaccines will require a better understanding of the influence of immune imprinting and exposure history on vaccine effectiveness (Knight 2020). 

Gaps 

• More information is needed regarding:
  • The underlying mechanisms of immune imprinting and the host and virologic factors that influence its outcome (Cobey 2017, Guthmiller 2018, Yewdell 2020, Zhang 2019). 
  • The role of immune imprinting from early childhood exposure on subsequent immune responses to influenza infection or vaccines (Dugan 2020, Worobey 2020). 
  • The role of immune imprinting on B cell responses to non-HA head antigens (e.g., NA- and HA-stalk antigens) and on T cell responses to internal proteins (Zhang 2019). 
  • How immune imprinting may protect or blunt responses to vaccination and its effect on vaccine innovation (Meade 2020, Coughlan 2018, Henry 2018). 
  • The role that annual vaccination plays in determining subsequent vaccine effectiveness (Belongia 2017, Kim 2020) such as potential alterations in B cell and CD4 T cell responses following repeated seasonal influenza vaccinations (Richards 2020), which may have implications for vaccine design (such as inducing immune responses more similar to those of natural infection or modifying antigenic structures and vaccine-adjuvant combinations). 

Issue: New immune correlates of protection for influenza vaccines 

Barriers 

• Identifying a single correlate of protection responsible to guide selection of target antigens and measuring protective efficacy of influenza vaccines may not be possible. Instead, multiple or complex correlates of protection may be needed, since several different antigens stimulate protective responses and the responses interact synergistically (Lim 2020, Plotkin 2020). 
• The most commonly used marker for immune response to influenza infection or vaccination is the serum HA antibody inhibition (HAI) titer; however, measurement of this titer has limitations in predicting vaccine effectiveness, particularly for non-HA strain-specific vaccines, and does not provide a comprehensive assessment of immunity (Reber 2013, Wraith 2020). 
• Immune markers that may correlate with protection but are not yet validated and require further assessment include: interferon gamma-secreting cells, cross-reactive CD8 and CD4 T cells in peripheral blood, serum NA inhibition (NAI) antibodies, nasal IgA, and anti-HA stalk antibodies (Krammer 2020). 
• A correlate of protection for assessing mucosal immunity (e.g., through measurement of mucosal antibodies) has not yet been established. 
• Immune correlates of protection may differ quantitatively and qualitatively by type of vaccine, various host characteristics (e.g., age, gender, major histocompatibility complex [MHC] group) and intended outcome of vaccination (e.g., prevention of mucosal or respiratory tract infection vs. prevention of severe disease), which complicates assessment approaches (Plotkin 2010). 

Gaps 

• The lack of qualified and harmonized methods to measure antibody indicators other than HAI remains an important gap to assessing immunity generated by natural infection or more broadly protective vaccines (FLUCOP, Krammer 2020, Lim 2020, Madsen 2020, Ng 2019, Reber 2013). 
• Additional priority needs include the following:
  • Sensitive and standardized methods to collect and measure antibodies at mucosal sites and identify correlates of protection for mucosal immunity (Reber 2013). This is particularly important for live-attenuated influenza vaccines (LAIVs), for which a correlate of protection has yet to be defined (Reber 2013, Shannon 2019). 
  • Reagents and standardized assays for non-HA head immune responses (such as HA-stem antibodies, NA antibodies, and cell-mediated immunity) to assess different types of humoral immunity and to allow comparison of data across studies (Coughlan 2018, Reber 2013). 
  • One or more standardized quantitative influenza-specific CD4 and CD8 assays to identify CD4 and functional cytolytic CD8 T cells elicited by natural infection and novel influenza vaccines that are correlated with protection from defined clinical end points (e.g., serious complications from influenza in older adults as well as potential vaccine-enhanced disease in relation to CD8 T cell stimulation). 
  • In vitro high-throughput assays to measure protective responses other than virus neutralization and T cell–mediated immune responses to understand the level of protective immunity contributed by these other responses (such as influenza virus–specific antibody-dependent cellular cytotoxicity [ADCC], antibody-dependent cellular phagocytosis, and complement dependent cytotoxicity) (Gianchecchi 2019, Plotkin 2018). 

Strategic Goals and Aligned Milestones for Topic 2: Immunology and Immune Correlates of Protection

Additional R&D Priorities for Topic 2: Immunology and Immune Correlates of Protection 

• Identify how distinct types of B cell responses (e.g., neutralizing antibodies against the HA head and ADCC antibodies against non-neutralizing conserved epitopes) synergize or compete to result in a protective response (Boudreau 2019, Coughlan 2018, Jegaskanda 2018). 
• Expand the use of new computational tools, such as multiplexing and systems immunology using innate signatures, to evaluate immunologic responses in various populations to identify vaccine-induced immunologic mechanisms and signatures predictive of protective immunologic responses (Nakaya 2016, Stacey 2018). 
• Enhance collaboration with selected research consortia focusing on other pathogens (e.g., HIV, coronaviruses) to share information on key determinants of long-lived immunity, such as intrinsic host factors or antigen delivery strategies. 
• Develop and test complex correlates of protection, potentially using the CHIVIM (Plotkin 2020). 
• Explore methods to circumvent virus escape of T cell immunity, such as strategies to optimize peptide presentation and boost responses to subdominant epitopes (Clemens 2018). 
• Investigate the role for influenza vaccination in priming hosts for adequate CD4 T cell support for neutralizing antibody production and for localized effector function in the lung to enhance vaccine protection against influenza. 
• Continue to research additional influenza virus epitopes (such as non-structural proteins) as potential vaccine targets (Sautto 2018), including generation of mAbs with a wide breadth of activity to identify potential epitopes that can be targeted by novel vaccines. 
• Clarify the role of CD4 T helper cells in the B cell response against influenza (Crank 2019). 
• Continue to determine how immune dysregulation contributes to severe influenza disease (Bouvier 2018, Erbelding 2018, Crowe 2019). 
• Further explore the potential mechanisms for viral interference or immune exclusion, such as ACE-2 receptor competition, interferon-mediated effects, or other mucosal defense mechanisms, which may provide information relevant to adjuvant design or immunization timing (Anchi 2020). 
• Assess how the route of vaccine administration affects the protective immune response in the various immunologic compartments (Morens 2019). 
• Apply mathematical modeling to explore the interplay of different immune system components.

Issue: Suboptimal vaccine effectiveness and annual reformulation of current seasonal influenza vaccines 

Barriers 

• Currently available seasonal influenza vaccines do not provide a high level of protection against matched strains, particularly in the elderly, show diminished effect against drifted strains, and likely would not protect against antigenically shifted strains (Gouma 2020a). 
• Because of the time needed to manufacture egg-based influenza vaccines each season, decisions on which strains to include need to be made months in advance, which allows the potential for later changes in circulating strains to result in antigenic mismatches between vaccine strains and circulating viruses, potentially reducing vaccine effectiveness (Zimmerman 2016). While a good match between vaccine strains and circulating strains has occurred during many influenza seasons, antigenic mismatches also occur (Trucchi 2019). 
• Because whole-virus influenza vaccines were noted to be reactogenic, current seasonal influenza vaccines are based on immune response to the HA head. Since this domain is constantly changing and evolving, vaccines that focus on HA head antigens may have limited vaccine effectiveness by the time they are available for clinical use. 
• The H3N2 subtype, which has been circulating in humans since the 1968 H3N2 pandemic, continues to undergo extensive antigenic and genetic changes that often result in reduced seasonal vaccine effectiveness against H3N2 subtype strains (Gouma 2020b, Flannery 2019). 
• Cell-mediated immune responses to current seasonal influenza vaccines, especially LAIVs, may be suboptimal because of a mismatch between T cell epitopes of cold-adapted master donor viruses isolated about 60 years ago and currently circulating viruses (Korenkov 2018). (Although influenza T cell epitopes are considered more conserved in comparison to B cell epitopes, some evolution can occur over time, which could lead to these mismatches.) 
• Adjuvants licensed for use in influenza vaccines include alum, MF59, AS03, and AF03 (Wei 2020). Adding adjuvants to vaccine formulations may improve the potency and breadth of humoral immune responses to seasonal vaccines, particularly in the very young or the elderly, but key barriers exist for developing vaccines that use novel adjuvants. The manufacturing costs may be high, and it can take years to develop a new adjuvanted vaccine. In addition, unforeseen adverse effects may surface during post-marketing assessment, which increases risk to manufacturers (Tregoning 2018). Adjuvants with good safety profiles, however, may improve vaccine effectiveness in children and the elderly and enable dose-sparing, which may be important for pandemic response. 
• Immunosenescence (a decline in immune function with aging) is a barrier to vaccine protection in the elderly (Castrucci 2018, Navarro-Torné 2019). 
• Influenza viruses generally do not involve a viremic phase, so the primary encounter with the immune system is at the mucosal level. Creating effective vaccines to respiratory viruses that lack a viremic phase may limit options for inducing durable protective immunity. 
• Success in implementing the clinical research needed to improve seasonal influenza vaccines may require enhanced cooperation and transparency, particularly sharing of data and clinical samples from industry and more rapid responsiveness from regulatory authorities. 
• NA-inhibiting antibodies may play an important role in vaccine effectiveness by reducing disease severity and providing cross-protection, but the NA antigen content in current seasonal influenza vaccines is unstandardized and generally low or unknown, or nonexistent (in recombinant influenza vaccines) (Eichelberger 2018, Eichelberger 2019, Giurgea 2020, Krammer 2018b, Yamayoshi 2019, Zheng 2020). In addition, current production processes may denature or destroy much of the NA that may be present in the formulations.

Gaps 

• Research is needed to:
  • Assess whether vaccine combinations (e.g., LAIV and IIVs) are synergistic in enhancing protection against influenza viruses (Yan 2018). 
  • Determine the utility of adjusting antigen doses for different populations and age-groups to enhance immunogenicity and vaccine effectiveness. 
  • Assess the level of NA-specific antibody titers necessary to induce protection against disease and to establish the dose of NA that reliably elicits a protective titer for use in seasonal vaccines (Chen 2018). 
  • Identify production processes that retain NA in vaccine formulations. 
  • Confirm and standardize the NA content in seasonal vaccines (Giurgea 2020). 
  • Determine whether NA-standardized products provide greater protective efficacy than current HA-based vaccines. 

Issue: The need to shorten the lag time for seasonal influenza vaccine development 

Barriers 

• The production time for seasonal vaccines using egg-based and recombinant cell-culture methods is at least 6 months from strain selection to vaccine production (Harding 2018, Paules 2019a). 
• Most of the world’s influenza vaccines are produced in eggs, which creates a major disincentive for manufacturers to change to other technologies that require building and validating new production facilities, unless such technologies substantially enhance efficacy or lower costs. Furthermore, egg-based methods don’t currently provide sufficient capacity to produce pandemic vaccines on a global scale. 
• Many influenza viruses do not grow well in eggs, which can result in egg-adapted mutations that alter antigenicity and reduce vaccine effectiveness (Allen 2018, Skowronski 2014, Zost 2017). Similar growth problems may also occur with cell-culture and recombinant methods. 
• Variant influenza viruses can emerge late in the vaccine production cycle, when it’s too late to change the vaccine composition for that season, increasing the likelihood of mismatches (WHO 2016a). Shortening the lag time will improve this issue but may not eliminate it. 

Gaps 

• Several needs are evident:
  • Data over time to determine whether vaccines generated through egg-independent processes may be significantly more effective, result in high yields, be cost-effective to produce, and not lead to other significant challenges as more experience is gained regarding their use. 
  • Research to develop mRNA-based technologies, similar to those used in several of the new COVID-19 vaccines, as an alternative platform (Bahl 2017, Bartley 2021, Feldman 2019, Pardi 2018, Scorza 2018). 
  • Studies to compare efficacy, safety, and effectiveness of new egg-independent production methods with each other and with traditional egg-based methods (Trombetta 2019).
  • A coordinated global effort to expand production capacity for egg-independent vaccine production technologies, including identifying creative funding strategies— provided these methods offer significant advantages. 

Issue: Seasonal influenza vaccines that prevent severe disease and are suitable for use in LMICs 

Barriers 

• Many LMICs do not have national influenza vaccine programs because of challenges with current seasonal vaccine delivery approaches, competing public health priorities, the need for annual revaccination, lack of information indicating that influenza is an important public health problem, and the moderate effectiveness of current vaccines in preventing infection and disease (WHO 2017). 
• Many LMICs are located in tropical and subtropical areas of the world, where influenza occurs year round, including times when there are gaps in vaccine availability owing to the need to reformulate vaccines each year (WHO 2017). 
• Production facilities are geared to cope with the annual demands for seasonal vaccine—largely in high- and upper-middle-income countries— and are inadequate to cope with the enhanced worldwide demand that would be created by a pandemic; the development of mobile manufacturing centers may help to address this barrier in LMICs. 
• More appropriate vaccine delivery methods, such as microarray patches or other needle-free injection approaches, suitable for LMICs, are needed to facilitate seasonal vaccination and pandemic response globally. 

Gaps 

• Important needs include:
  • Data on vaccine effectiveness when influenza vaccines and COVID-19 vaccines are co-administered, as well as on vaccines that combine antigens for the two diseases. 
  • Improved seasonal vaccines that offer better protection against or attenuation of severe influenza disease and more durable protection (WHO 2017). 
  • Data to demonstrate the public health burden of influenza—especially severe influenza—in LMICs. (Also see Topic 6: Policy, Financing, and Regulation.) 
  • Information on whether influenza vaccines prevent severe acute respiratory infection (SARI) and, if so, what viral, host, or seasonal factors contribute to better clinical outcomes (Bouvier 2018), as well as information on better ways to assess SARI. 
  • Methods to reliably assess vaccine protection against severe disease including standardized criteria and measurements of disease severity. 
  • Determining whether or not certain genes are correlated with severity of influenza (Allen 2017, Randolph 2017) and how this information may relate to vaccine response versus natural infection. 
  • Determining if expanding use of new vaccines in young children can reduce influenza transmission and thereby lower the overall burden of the disease.

Strategic Goals and Aligned Milestones for Topic 3: Vaccinology for Seasonal Influenza Vaccines

Additional R&D Priorities for Topic 3: Vaccinology for Seasonal Influenza Vaccines 

• Develop stable cold chain–independent vaccine technologies suitable for stockpiling and deployment in low-resource settings. 
• Continue to develop and implement methods for growing influenza viruses (particularly H3N2 strains) in eggs that reduce the likelihood of egg-adaptive mutations during production (Wu 2019). 
• Continue to conduct clinical studies to enhance the use of existing seasonal influenza vaccines, including improvements in production, duration, and breadth of protection; safety and immunogenicity profiles; and dose-sparing formulations, especially for high-risk groups such as pregnant women, immunosuppressed patients, obese people, the very young, and the very old. 
• Determine the optimal frequency of seasonal influenza vaccination, given both scientific and logistical constraints, and clarify if this is similar for LMICs and other countries. 
• Enhance understanding of viral pathogenicity, host factors, and progression to severe disease, which may guide novel strategies for improved seasonal vaccines (Erbelding 2018, Mettelman 2020). 
• Develop protocols to standardize the timing, specimen collection, and methods of data analysis used in large-scale studies of seasonal vaccine responses (Stacey 2018). 
• Continue to research the role that certain genes play in disease severity for influenza, which has important implications for developing and evaluating new vaccine candidates. 
• Consider formalizing a plan for prospective meta-analysis of vaccine effectiveness studies, with a focus on test-negative designs as a commonly used approach. 
• Replicate the preliminary UK study of direct and indirect effects of LAIV in children under 12 years of age (Pebody 2018). 
• Conduct vaccine probe studies to ascertain the contribution of influenza to particular clinical syndromes or the overall disease burden in children. 
• Promote development of new vaccines that are stable at 4⁰C (39°F) or warmer for a considerable period so they can be used in low-resource settings. 

Issue: New constructs for broadly protective and durable influenza vaccines 

Barriers 

• There are many possible vaccine platforms and candidates for broadly protective or universal influenza vaccines, but resources for conducting clinical trials are limited, necessitating the selection of promising candidates for advanced trials (Epstein 2018, Madsen 2020). 
• Inter-laboratory variability in assay techniques and determination of assay end points limit comparisons among measurements of immunogenicity of broadly protective or universal influenza vaccines in clinical trials (Reber 2013, Stacey 2018). 
• The heterogeneity of HLA types within populations may complicate the development of new vaccine technologies involving T cell– directed approaches (Clemens 2018, Wen 2021). 
• Global strategies for producing pandemic influenza vaccines and ancillary supplies to mitigate an emerging pandemic are based on using the commercial seasonal influenza vaccine manufacturing infrastructure. In the absence of a truly universal influenza vaccine that is used globally to mitigate pandemic threats, development of broadly protective vaccines should, whenever possible, select manufacturing platforms that can be quickly adapted to respond to pandemics. 

Gaps 

• Novel influenza vaccines based on mechanisms of protection other than neutralizing antibody may not prevent all infections (and allow some degree of transmission), but may decrease disease severity. Public health officials and regulators need to address the benefits of preventing severe disease versus reducing transmission of infection (Epstein 2018). 
• Universal influenza vaccine constructs may need to include multiple antigenic targets to enable broadly protective and durable immunity against a wide range of influenza viruses (Jang 2020, Lopez 2020), including conserved antigens targeted by T cells, given clinical trial findings that broad T cell responses are associated with asymptomatic or mild disease. 
• Other needs include:
  • Additional correlates of protection to assess novel influenza vaccine technologies (Madsen 2020). (Also see Topic 2: Immunology and Immune Correlates of Protection.) 
  • Guidance on the use of correlates of protection and clinical end points in assessing broadly protective or universal influenza vaccines at different stages of development. 
  • New approaches for vaccine/immunogen design to achieve robust immune responses to conserved regions of the influenza virus (Kanekiyo 2019, Kanekiyo 2020, Krammer 2016, Nachbagauer 2018, Ross 2019, Yamayoshi 2019), which may require identifying successful strategies for overcoming the immunodominance of the HA globular head domain (Amitai 2020, De Jong 2020). 
  • More clarity about potential safety issues, such as enhancement of infection or enhanced lung pathology, with next-generation influenza vaccines that do not protect against receptor binding (e.g., that elicit antibodies to stem immunogens). 
  • Better understanding of the role of stem-specific antibodies in viral fitness and immune escape. 
  • Mechanisms for evaluating and comparing data obtained from preclinical and clinical studies—including data from commercial entities—to refine and optimize platforms and strategies for durable, broadly protective or universal vaccines. 
  • Research to better understand how influenza viruses will respond to new selective pressures created by broadly protective or universal influenza vaccines. 
  • More research to evaluate the potential role of vaccine mechanisms other than those that achieve sterilizing immunity (e.g., induction of non-neutralizing antibodies and cell-mediated immunity) in promoting clinical outcomes such as cross-protection against future variant strains. 
  • More research to assess the potential for T cell–based or non-neutralizing-antibody– based vaccines to exacerbate influenza immunopathology and allow considerable influenza viral replication or promote vaccine-associated enhancement of respiratory disease (VAERD). 

(Also see section on regulatory challenges for development of broadly protective or universal influenza vaccines in Topic 6: Policy, Financing, and Regulation.) 

Strategic Goals and Aligned Milestones for Topic 4: Vaccinology for Broadly Protective or Universal Influenza Vaccines

Additional R&D Priorities for Topic 4: Vaccinology for Broadly Protective or Universal Influenza Vaccines 

• Continue to research novel mechanisms, antigens, and platforms to develop new influenza vaccine candidates. 
• Expand options for multidisciplinary research for influenza vaccine R&D, including collaboration with computational biologists and veterinary researchers. 
• Investigate how influenza viruses will respond to new selective pressures created by broadly protective influenza vaccines, particularly as promising candidates advance to clinical trials. 
• Conduct ongoing research to evaluate the potential role of vaccine mechanisms other than those that achieve sterilizing immunity to promote clinical outcomes, such as cross-protection against variant influenza virus strains. 
• Explore the potential for combination antigen vaccines to achieve broad immunologic protection (Epstein 2018, Estrada 2019). 
• Continue to monitor and track universal influenza vaccine candidates in clinical and preclinical development, including technologies such as existing and novel adjuvants and alternative delivery methods such as oral formulations and microneedle patches (Ostrowsky 2020). 

Issue: Optimization of animal models used for influenza vaccine R&D 

Barriers 

• The use of animal models that accurately reflect disease and immune responses in humans is an important element of influenza vaccine R&D (D’Alessio 2018, Lane 2020). A number of animal models exist for studying influenza, such as mice, ferrets, guinea pigs, rats, hamsters, swine, and nonhuman primates (NHPs). While these animals can provide extremely valuable information, they also have important limitations. For example, influenza disease in certain animal models does not accurately mimic influenza disease in humans. Also, important immunologic differences exist between humans and certain animals (Bouvier 2010, D’Alessio 2018, Lane 2020). 
• NHPs may be useful for studying specific aspects of vaccine protection, but ethical considerations and cost constrain their use. 
• Duration of protection is difficult to study in animals, particularly if protection is due to broadly protective antibodies or other immune mechanisms. 
• Because influenza viruses are constantly changing and evolving, up-to-date and representative viral stocks must be readily available for influenza vaccine research in animal models. 
• Humans have pre-existing immunity to influenza viruses, which can be difficult to mimic in animals, so pre-exposure animal models are needed to address this issue, but they are expensive to maintain and reinfection can be challenging (D’Alessio 2018). 
• Different vaccine platforms may require evaluation in different animal models, which adds complexity to determining preclinical vaccine research approaches (Lane 2020). 
• Head-to-head studies with multiple vaccine candidates could help to better understand vaccine-induced immunity, but securing needed study materials can be challenging. 

Gaps 

• Needs related to animal models include:
  • Standardized and harmonized animal models to evaluate and compare vaccine-related research studies. Parameters to consider include the challenge virus strain, dose, route, and volume; the animal test species and appropriate clinical end point for that species; and the specific pathogen-free status of animals under study (D’Alessio 2018). 
  • Ongoing efforts to ensure that validated, reliable reagents, updated viral strains and stocks, and harmonized assays are available to the research community to improve understanding of the innate and adaptive immune responses in ferrets and other small mammals, such as hamsters and guinea pigs (Albrecht 2018). 
  • Additional research to optimize animal models for examining: (1) specific issues such as cell-mediated immunity and enhanced respiratory disease; (2) novel vaccine platforms for broadly protective or universal vaccines; and (3) additional factors that may be important for experimental vaccine design, such as tissue tropism and patterns of receptor distribution (D’Alessio 2018, Margine 2014). 
  • Further refinement of animal models to mimic different human conditions such as underlying morbidities, advanced age, immunocompromised status, and pregnancy. 
  • Efforts to ensure ongoing back-translation of data from humans to animal models. 
  • More data on the role of the microbiome in animal research for influenza vaccines. 
  • Efforts to characterize the MHC epitopes in animal models, particularly ferrets, to better allow assessment of T cell-based vaccines. 
  • Efforts to advance emerging technologies to reduce reliance on animal models. 
  • Better understanding of protection-relevant antibody subclasses and how they compare between animals and humans. 

Issue: Development and refinement of the CHIVIM 

Barriers 

• Human infection studies are limited to healthy adults without comorbidities and thus do not reflect potential outcomes in special populations (Sherman 2019).
• These studies can assess only mild disease and viral shedding but not severe disease (Sherman 2019). 
• Human infection studies may not necessarily mimic community-acquired infections because of artificialities in the model (i.e., studies are conducted under controlled situations) (Innis 2019a). 
• Human infection studies have limited statistical power because of relatively few subjects. 
• Ethical and safety concerns have constrained the use of such studies (Sherman 2019). 
• Challenge viruses must be updated frequently, which can be expensive and time consuming, since people can become immune to such viruses through exposure to seasonal influenza viruses in the community. 
• Current models do not use novel or non-seasonal influenza strains, owing to safety considerations. It may be possible to reassess this approach. 
• Longer-term human infection studies that may be needed for examining long-term immunity are more difficult to fund and require longer-term retention of participants. 

Gaps 

• Key issues regarding the CHIVIM need to be addressed, such as lack of standardization for certain elements of the model; limited access to challenge viruses; lack of harmonized protocols; better definition of end points, particularly for determining mucosal immunity; the need for agreed-upon criteria for selection of challenge strains; regulatory challenges; environmental considerations such as heating, ventilation, and air conditioning (HVAC); concerns about dual use for published sequences; and concerns regarding risk to study participants and the community (Innis 2019a). It is also important to capture and standardize metadata to enable cross-study analysis and to address a greater diversity of research questions. 
• Other needs include:
  • Efforts to include subjects of diverse age ranges and past experience with influenza to better study impacts of immunity induced by vaccine and natural infection. 
  • Resources to expand human infection studies to additional research sites to ensure greater global diversity in the research (Erbelding 2018). 
  • Steps to tie research in CHIVIM studies to research in animal models and vice versa. 
  • Human challenge models that assess breadth of protection for broadly protective vaccines. 
  • Further research to explore how best to apply systems biology approaches to influenza vaccine studies involving human infection models (Sherman 2019).

Strategic Goals and Aligned Milestones for Topic 5: Animal Models and the Controlled Human Influenza Virus Infection Model

Additional R&D Priorities for Topic 5: Animal Models and the Controlled Human Influenza Virus Infection Model 

• Determine additional factors that may be important considerations for preclinical research, such as tissue tropism and patterns of receptor distribution (D’Alessio 2018, Margine 2014). 
• Identify appropriate animal models for studying specific research topics, such as virus transmission, cell-mediated immunity, and enhanced disease (D’Alessio 2018, Margine 2014). 
• Continue to assess animal models to ensure genotypic and phenotypic diversity to better mimic human conditions. 
• Conduct further research to determine the role of NHP models for influenza vaccine R&D (Davis 2015). 
• Conduct further research on the role of the microbiome in animals for influenza vaccine R&D. 
• Explore ways to leverage emerging technologies to reduce reliance on animal models. 
• Ensure that veterinary specialists who are certified in laboratory animal medicine are included in research and strategic discussions involving the development of new animal models. 
• Ensure ongoing availability of appropriate and standardized influenza viruses for animal model research and for the CHIVIM, using current and representative viruses. 
• Develop ongoing mechanisms to ensure back-translation of research findings from humans to animal models, in both CHIVIM studies and clinical studies. 
• Explore development of human infection models that use novel viruses to better assess breadth of protection for broadly protective influenza vaccines. 
• Encourage collaboration between groups developing viruses for human infection models and those developing the models. 
• Continue to develop ways to integrate systems biology approaches with human infection studies to promote understanding of influenza pathogenesis and immunology (Sherman 2019). 

Issue: Influenza vaccine investment strategies 

Barriers 

• Substantial financial risks and inadequate incentives create significant barriers to bringing new broadly protective influenza vaccines to market (Osterholm 2012). 
• Definitive human efficacy data from large comparative trials are needed to determine noninferiority or superiority of next-generation influenza vaccines; such trials are difficult to conduct because of high costs and organizational barriers. 
• Bringing vaccines to market requires crossing what is referred to as the “valley of death” for vaccine development. This period encompasses early clinical trials through phase 3 clinical trials to the point of regulatory approval and early commercialization. During this time, substantial research, development, and licensure costs are incurred while outcomes are uncertain and no revenue is generated (Osterholm 2012). A root cause of the “valley of death” for new vaccines is that an asymmetry of risk exists, as manufacturers take on most of the risks but the public sector does not balance the risk with commitments and funding. 
• Transformative changes in influenza vaccines will require changing the basic manufacturing infrastructure—currently dominated by a few large companies—that drives influenza vaccine production in the private sector, which is currently based on the assurance of an ongoing annual seasonal vaccine market (Osterholm 2012). 
• Development and deployment of improved influenza vaccines are occurring within the context of an existing standard of care in high-income countries (e.g., 30% to 40% of the US population is immunized against influenza annually) and lack of access to influenza vaccines in areas of the world, including those with a high burden of influenza. 

Gaps 

• A coordinated commitment to sustained (i.e., 10 years) funding of R&D for universal influenza vaccines is lacking, leaving the world vulnerable to the next influenza pandemic (Hayes 2020). Efforts are needed to consider the feasibility of a “mission-driven” approach to R&D for universal influenza vaccines through a new public-private partnership, akin to COVID-19 vaccine development (Sabin 2021). Plans are also needed for ongoing funding of basic science research into influenza virology and immunology. 
• Leveraging public investment to share the risk with vaccine developers will be critical for implementing clinical trials, completing advanced development, and manufacturing of all meritorious universal influenza vaccine candidates. 
• The predominant commercial model for global influenza vaccine production is based on annual reformulation of influenza vaccines and on reliance of egg-based technology for production, and the companies that profit from this model may resist change. To overcome this, innovative strategies need to be considered for financing development of new influenza vaccines. Such strategies should also address the interests and financial constraints of LMICs (Sabin 2019). 
• Additional efforts are needed to articulate for policy makers and funders the global value of developing improved influenza vaccines and the risk of not doing so (Bartsch 2020, Bloom 2018, Global Funders Consortium 2018). These efforts should consider issues such as which types of vaccines are most suitable to which populations and the different use cases for vaccines. 
• A coordinated communication strategy must advocate for the need to improve influenza vaccines and to stimulate investment in innovative R&D strategies. One issue is that influenza is often seen as nothing more than a mild illness, and key messages need to balance the threat of annual influenza epidemics with the danger of pandemic influenza. 
• A comprehensive picture that details influenza vaccine funding trends is not available, which creates challenges in identifying research gaps and ensuring coordination of funding (Global Funders Consortium 2017). 
• A number of influenza vaccine technologies are under development, and efforts to track their progress are under way (Ostrowsky 2020). Additional efforts, however, must analyze the vaccine landscape and identify the paths that vaccine developers in industry, academia, and government laboratories can take—as well as to identify the potential pitfalls—to stimulate and maintain productive vaccine R&D. 
• The COVID-19 pandemic can help reframe expectations regarding the global impact of a pandemic respiratory virus and the value of vaccines in preventing the potentially devastating effects. A value proposition for influenza vaccines should capitalize on the importance of pandemic preparedness, which may stimulate the necessary support. 
• Efforts are needed to enlist scientists who are not currently focused on influenza to enhance innovation and generate multiple pathways toward creating improved influenza vaccines (Sabin 2019), including potential synergies with COVID-19 vaccine R&D that involve similar vaccine technologies and manufacturing processes. Scientists in other relevant fields, such as mucosal immunology and experimental animal challenge, and those who work with non-influenza respiratory viruses should be engaged to provide “out of the box” thinking. 

Issue: Enhanced coordination and information sharing 

Barriers 

• Influenza vaccine R&D efforts to date, while substantial, have been relatively fragmented, with a lack of coordinated effort and shared vision among a diverse range of stakeholders (Sabin 2019). 
• The lack of adequate data management and sharing among academic, industry, and government developers inhibits influenza vaccine development. 
• Challenges with mapping and potentially sharing of intellectual property and proprietary technologies is a barrier to combining approaches that could better address the public health need for improved influenza vaccines. 
• Under the WHO Pandemic Influenza Preparedness (PIP) Framework, which establishes a global system for sharing viruses with pandemic potential and ensuring access to vaccines and antivirals during a pandemic (WHO 2011), the GISRS and GISAID have successfully fostered international sharing of influenza virus isolates and gene sequences for years. However, restrictions on the use of influenza viruses could result under certain provisions of the Nagoya Protocol (United Nations Environmental Programme, Secretariat of the Convention on Biological Diversity 2011). 

Gaps 

• Developing innovative vaccine approaches will require substantial additional investments in R&D and vaccine production infrastructure, particularly to overcome challenges encountered during the “valley of death” period. Coordinated partnerships involving national governments, the pharmaceutical industry, philanthropic organizations, and academia will be critical to advance such vaccines through clinical trials and licensure (Bresee 2019, Osterholm 2012). 
• Improved coordination is needed to maximize the value of influenza vaccine research, such as exploring options for reuse of influenza vaccine study data. 
• The need for developing, adopting, and establishing data standards for certain research areas and identifying public data repositories for housing and sharing data should be further assessed. 
• The impact, if any, of the Nagoya Protocol on influenza vaccines R&D should be assessed on an ongoing basis, including the impact of national Access and Benefit Sharing (ABS) legislation. 

Issue: Regulatory challenges for development of broadly protective or universal influenza vaccines 

Barriers 

• Issues around licensure of broadly protective or universal influenza vaccines are complex and will require development of new tools, such as new potency assays and new correlates of protection or immune markers likely to predict protection. 
• Different vaccine goals—such as short- vs. long-term protection, protection against severe vs. mild disease, or practical approaches vs. the most efficacious approaches—may require different clinical trial designs, which poses additional challenges for clinical evaluation. 
• While the general licensing approach is the same for all vaccines, specific regulatory issues for any new influenza vaccine will depend on the particular construct of the vaccine and will be unique to that situation; therefore, it is not possible to create a “one size fits all” regulatory pathway for next-generation influenza vaccines. 
• New influenza vaccines may involve approaches for which safety is poorly characterized—such as use of new antigens, new delivery systems, or novel platform technologies—necessitating careful safety assessments throughout the R&D process. 

Gaps 

• Needs in this area include:
  • Further clarification regarding what the regulatory requirements will be for licensure of new influenza vaccines that do not rely on current standards (e.g., the HAI assay). Currently, there is no regulatory pathway for approval and deployment of universal influenza vaccines. Examples of key issues that need resolution include requirements for demonstrating efficacy (e.g., against future pandemic viruses), defining criteria for “universal” vaccines, identifying requirements for post-marketing studies, and determining a mechanistic correlate of protection. 
  • Regulatory guidance to clarify how data from human infection studies can be used to support licensure of next-generation influenza vaccines (Innis 2019a). 
  • A better understanding of the correlates of protection for new influenza vaccines, how they can be measured, and how they will be used for licensure. (Also see Topic 2: Immunology and Correlates of Protection.) 
  • Enhanced manufacturing capabilities, new methods of vaccine characterization, and regulatory issues related to these processes need to be identified and addressed. 
  • Improved infrastructure for post-licensure monitoring of new influenza vaccines, particularly if they are licensed under alternative regulatory pathways. 
  • Further guidance from regulatory authorities to establish robust measures of vaccine effectiveness that would enable comparisons among vaccines or combinations of vaccines.

Strategic Goals and Aligned Milestones for Topic 6: Policy, Financing, and Regulation

Additional R&D Priorities for Topic 6: Policy, Financing, and Regulation 

• Ensure ongoing and substantial funding for basic science research into influenza virology, immunology, and vaccine R&D. 
• Explore strategies for improving efficiencies of phase 3 clinical trials for universal influenza vaccines, such as harmonizing protocols on issues such as common end points and common validated assays, to allow data to either be combined or compared effectively. 
• Continue to promote ongoing communications among international regulators regarding the development of universal influenza vaccines, including exploration of regulatory harmonization.

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Appendix 1: See pdf for a list of steering group and taskforce members involved during the IVR development phase.

Appendix 2: Definitions and Key Features of Universal Influenza Vaccines

Appendix 3: Summary of Influenza Vaccines R&D Roadmap High-Priority Milestones by Topic and Strategic Goal

Download the Roadmap