Vaccination and the evolution of seasonal influenza

Although vaccines against seasonal influenza are designed to protect against currently circulating strains, they may also affect the emergence of antigenically divergent strains and thereby change the rate of antigenic evolution. Such evolutionary effects could change the benefits that vaccines confer to vaccinated individuals and to the host population (i.e., the private and social benefits of vaccination). To investigate the potential evolutionary impacts of vaccination, we simulated the dynamics of an influenza A/H3N2-like pathogen in a host population receiving annual vaccines. On average, increasing vaccination rates decreased the cumulative amount of antigenic evolution of the viral population and the incidence of disease. Vaccination at a 5% random annual vaccination rate, implying a 48% cumulative vaccine coverage after 20 years, decreased cumulative evolution by 56% and incidence by 76%. These effects were mediated by the breadth of immunity conferred by the vaccine. To understand how the evolutionary effects of vaccination might affect its private and social benefits over multiple seasons, we fit linear panel models to simulated longitudinal infection and vaccination histories. Including the evolutionary effects of vaccination lowered the private benefits by 14% but increased the social benefits by 30% (at a 5% annual vaccination rate) compared to when evolutionary effects were ignored. Thus, in the long term, vaccines9 private benefits may be lower and social benefits may be higher than predicted by current measurements of vaccine impact, which do not capture long-term evolutionary effects. These results suggest that conventional seasonal vaccines against influenza, if protective against transmission, could greatly reduce the burden of disease by slowing antigenic evolution. Additionally, these evolutionary effects could compound collective action problems, increasing the importance of social policies to encourage vaccination.