Fig. 4: Effect of prior immunity on epidemic incidence.

Within each subtype, the incidence for individual epidemics was log-transformed and subtracted by the city-specific mean of log incidence, to allow for comparison between cities. Antigenic variant-specific cumulative incidence was measured relative to the city-specific mean epidemic size, where 1 is equivalent to the mean epidemic incidence. r- and P values are from Pearson’s correlation tests (n = 37, 20, 9 and 45 for B/Vic, B/Yam, A/H1sea and A/H3, respectively). Antigenic variants of B/Yam rarely initiated multiple epidemics during the study period, and it was not possible to calculate a correlation coefficient for A/H1sea because the one new antigenic variant to emerge during the study period caused only a single epidemic per city.