Figure 3: An increase in the susceptible pool explains sudden changes in seasonality.

(A) Predicted effects of the proportion of infected $i\left(0\right)$ and susceptible $S\left(0\right)$ at the beginning of season on center of gravity. Points represent the estimated values for $i\left(0\right)$ and $s\left(0\right)$ between 2013 and 2019, showing the last two digits of a given year. The white vertical dashed line represents the $i\left(0\right)$ value used for simulating epidemic dynamics in panel B. (B) Changes in epidemic trajectories caused by an increase in the susceptible proportion at the beginning of season for a fixed value of $i\left(0\right)$. (C-F) Comparisons of interpolated transmission rates used for simulating the SIRS model, corresponding to each corner in Panel G. Black lines represent the transmission rates used for simulations. Gray lines represent the estimated transmission rates the Honshu island as a visual reference. (C) The estimated transmission rates for the Honshu island. (D) The resulting transmission rates for the Honshu island with equal amplitude as the Kyushu island. (E) The resulting transmission rates for the Kyushu island with equal amplitude as the Honshu island. (F) The estimated transmission rates for the Kyushu island. (G) Differences in peak epidemic timing when we increase the the susceptible proportion at the beginning of season from 7.8% to 10.5% using transmission patterns that interpolate the estimates for Honshu and Kyushu islands.