Influenza B and two subtypes of influenza A, H3N2 and H1N1, cause annual epidemics of influenza in humans. It has long been observed that the relative contribution of these types and subtypes varies from season to season. A recent model proposes that epochal evolution within subtypes may account for interannual variability in attack rates, and thus offers one explanation of this pattern. However, challenge experiments in animals as well as historical patterns of subtype replacement in humans suggest that cross-immunity between subtypes and types of flu may be dynamically important. We investigated 37 years of serologically confirmed influenza cases from St. Petersburg, Russia, to determine whether there is evidence for competition among H3N2, H1N1, and influenza B. A stochastic time series model was fitted to the data.
Results/Conclusions

We find unexpected differences between the dynamics of the three major serotypes of influenza and suggestive evidence of cross-immunity between types. This result will be useful for understanding future patterns in strain dominance.