Vaccination and the evolution of seasonal influenza

1Although vaccines against seasonal influenza are designed to protect against circulating strains, by affecting the emergence and transmission of antigenically divergent strains, they might also change the rate of antigenic evolution. Vaccination might slow antigenic evolution by increasing immunity, reducing the chance that even antigenically diverged strains can survive. Vaccination also reduces prevalence, decreasing the supply of potentially beneficial mutations and increasing the probability of stochastic extinction. But vaccination might accelerate antigenic evolution by increasing the transmission advantage of more antigenically diverged strains relative to less diverged strains (i.e., by positive selection). Such evolutionary effects could affect vaccinations direct benefits to individuals and indirect benefits to the host population (i.e., the private and social benefits). To investigate these potential impacts, we simulated the dynamics of an influenza-like pathogen with seasonal vaccination. On average, more vaccination decreased the rate of viral antigenic evolution and the incidence of disease. Notably, this decrease was driven partly by a vaccine-induced decline in the rate of antigenic evolution. To understand how the evolutionary effects of vaccines might affect their social and private benefits, we fitted linear panel models to simulated data. By slowing evolution, vaccination increased the social benefit and decreased the private benefit. Thus, in the long term, vaccinations potential social and private benefits may differ from current theory, which omits evolutionary effects. These results suggest that conventional seasonal vaccines against influenza, if protective against transmission and given to the appropriate populations, could further reduce disease burden by slowing antigenic evolution.