Epidemic Influenza — Responding to the Expected but Unpredictable

Abstract

In the United States, influenza viruses can be counted on to cause outbreaks sometime between fall and spring each year. However, the timing and severity of these epidemics and the distribution of circulating viruses are highly variable and difficult to predict (see figure). For example, according to the Centers for Disease Control and Prevention (CDC), the estimated number of influenza-associated deaths varies from 3000 to 48,000 during a seasonal U.S. outbreak. The past two seasons exemplify this unpredictability. The 2011–2012 U.S. influenza season was one of the mildest in the past two decades, even though influenza A(H3N2) viruses — a . . .