Numerous modern technological and scientific advances have changed the vaccine industry. However, nearly 70 years of influenza vaccine usage have passed without substantial changes in the underlying principles of the vaccine. The challenge of vaccinating against influenza lies in the constantly changing nature of the virus itself. Influenza viruses undergo antigenic evolution through antigenic drift and shift in their surface glycoproteins. This has forced frequent updates of vaccine antigens to ensure that the somewhat narrowly focused vaccine-induced immune responses defend against circulating strains. Few vaccine production systems have been developed that can entertain such constant changes. Although influenza virus infection induces long-lived immunologic memory to the same or similar strains, most people do not encounter the same strain repeatedly in their lifespan, suggesting that enhancement of natural immunity is required to improve influenza vaccines. It is clear that transformative change of influenza vaccines requires a rethink of how we immunize. In this study, we review the problems associated with the changing nature of the virus, and highlight some of the approaches being employed to improve influenza vaccines.

Pandemic influenza A viruses can emerge from swine, an intermediate host that supports adaptation of human-preferred receptor-binding specificity by the hemagglutinin (HA) surface antigen. Other HA traits necessary for pandemic potential are poorly understood. For swine influenza viruses isolated in 2009–2016, gamma-clade viruses had less stable HA proteins (activation pH 5.5–5.9) than pandemic clade (pH 5.0–5.5). Gamma-clade viruses replicated to higher levels in mammalian cells than pandemic clade. In ferrets, a model for human adaptation, a relatively stable HA protein (pH 5.5–5.6) was necessary for efficient replication and airborne transmission. The overall airborne transmission frequency in ferrets for four isolates tested was 42%, and isolate G15 airborne transmitted 100% after selection of a variant with a stabilized HA. The results suggest swine influenza viruses containing both a stabilized HA and alpha-2,6 receptor binding in tandem pose greater pandemic risk. Increasing evidence supports adding HA stability to pre-pandemic risk assessment algorithms.

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The number of new and improved human viral vaccines licensed in recent years contrasts sharply with what could be termed the golden era (1955‐1990) when vaccines against polio‐, measles, mumps, rubella, and hepatitis B viruses first became available. Here, we attempt to explain why vaccines, mainly against viruses other than human immunodeficiency virus and hepatitis C virus, are still unavailable. They include human herpesviruses other than varicella‐zoster virus, respiratory syncytial and most other respiratory, enteric and arthropod‐borne viruses. Improved oral poliovirus vaccines are also urgently required. Their unavailability is attributable to regulatory/economic factors and the properties of individual viruses, but also to an absence of relevant animal models and ethical problems for the conduct of clinical of trials in pediatric and other critical populations. All are portents of likely difficulties for the licensing of effective vaccines against emerging pathogens, such as avian influenza, Ebola, and Zika viruses.

To optimize the public health response to coronavirus disease 2019 (COVID-19), we must first understand the antibody response to individual proteins on the severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) and the antibody’s cross reactivity to other coronaviruses. Using a panel of 37 convalescent COVID-19 human serum samples, we showed that the magnitude and specificity of responses varied across individuals, independent of their reactivity to seasonal human coronaviruses (HCoVs). These data suggest that COVID-19 vaccines will elicit primary humoral immune responses in naïve individuals and variable responses in those previously exposed to SARS-CoV-2. Unlike the limited cross-coronavirus reactivities in humans, serum samples from 96 dogs and 10 cats showed SARS-CoV-2 protein-specific responses focused on non–S1 proteins. The correlation of this response with those to other coronaviruses suggests that the antibodies are cross-reactive and generated to endemic viruses within these hosts, which must be considered in seroepidemiologic studies. We conclude that substantial variation in antibody generation against coronavirus proteins will influence interpretations of serologic data in the clinical and veterinary settings.