Vaccination and antigenic drift in influenza

Abstract

The relationship between influenza antigenic drift and vaccination lies at the intersection of evolutionary biology and public health, and it must be viewed and analyzed in both contexts simultaneously. In this paper, 1 review what is known about the effects of antigenic drift on vaccination and the effects of vaccination on antigenic drift, and I suggest some simple ways to detect the presence of antigenic drift in seasonal influenza data. If antigenic drift occurs on the time scale of a single influenza season, it may be associated with the presence of herd immunity at the beginning of the season and may indicate a need to monitor for vaccine updates at the end of the season. The relationship between antigenic drift and vaccination must also be viewed in the context of the global circulation of influenza strains and the seeding of local and regional epidemics. In the data sets I consider--from New Zealand, New York, and France--antigenic drift can be statistically detected during some seasons, and seeding of epidemics appears to be endogenous sometimes and exogenous at other times. Improved detection of short-term antigenic drift and epidemic seeding would significantly benefit influenza monitoring efforts and vaccine selection.

Keywords: antigenic drift; evolution; haemagglutinin; influenza; vaccination.

Figure 1

Schematic diagram of antigenic drift as a function of host immunity, as predicted by the theory presented in ? ]. Vertical axis represents the mean distance between strains isolated at the beginning of the epidemic and strains isolated at the end of the epidemic. The amount of observed antigenic drift increases as immunity in the host population increases and pressures the virus population to evolve. In this example, the gray-shaded area indicates that herd immunity is higher than 80%, meaning that, on average, each host is more than 80% immune; in this part of the graph, there is insuffcient potential to transmit the virus among hosts (basic reproduction ratio < 1) and there is no epidemic. The absence of an epidemic implies that there is no viral reproduction and no viral evolution. In reality, if the basic reproduction ratio were truly less than unity, a small amount of disease transmission could still occur, allowing for some viral evolution.