Pandemic influenza dynamics and the breakdown of herd immunity.

Few if any attempts have been made to derive forecasts for the ongoing H1N1 pandemic as extrapolated from knowledge of seasonal influenza. Even simple back-of-the-envelope calculations are lacking. In this note we use first-approximation parameter estimates for the SIR model to compare seasonal and pandemic influenza, and then explore the implications of the existing classical epidemiological theory. In particular, we note the dramatic nonlinear increase in attack rate as a function of the percentage of susceptibles initially present in the population. This has severe consequences for the pandemic, given the general lack of immunity in the global population.