Short-term heterologous immunity after severe influenza A outbreaks

Conventional wisdom holds that influenza A and B are such genetically dissimilar viruses that infection with one cannot confer cross-immunity to the other. However, our examination of the records of the past 25 influenza seasons in the U.S. reveals that almost every time there is an early and severe influenza A outbreak, the annual influenza B epidemic is almost entirely suppressed (and is never suppressed otherwise). Temporary broad-spectrum (aka “heterologous”) immunity in the aftermath of influenza infection is the most direct explanation for this phenomenon. We find a remarkably weak degree of temporary cross-immunity is needed to explain these patterns, and that indeed influenza B provides an ideal setting for the observation of heterologous immune effects outside of the carefully controlled environment of a laboratory.