Effect of Social Distancing Caused by Heavy Snowfall on Seasonal Influenza Epidemic in a Local Region in Japan.

Background: The effectiveness of social distancing in controlling influenza epidemics is debatable. We analyzed the local impact of large-scale social distancing caused by heavy snowfall on influenza epidemic.

Methods: Surveillance data from the National Institute of Infectious Diseases on influenza diagnosed at registered Fukui Prefecture institutions were plotted in epidemic curves. We also compared the number of elementary and junior high school students suspended attendance due to influenza during the 2017/2018 season between the northern and southern Fukui Prefecture territories. Weekly incidence influenza rates and average rates before, during and after school closure caused by heavy snowfall in the northern territory were compared with those of the southern territory using Poisson generalized estimating equations to account for school variability.

Results: The northern territory epidemic curve, like the nationwide trend, rapidly declined during the heavy snowfall and formed a second peak 4 weeks later. The southern territory's curve was like the nationwide trend throughout. Weekly incidence rate ratios in schools for the northern versus southern territories varied from 0.23 during heavy snowfall to 2.32, 4 weeks after the snowfall. Cumulative incidence ratio (95% confidence interval) of the northern-to-southern territories was 0.92 (0.74-1.13; P = 0.42). School closure significantly reduced the incidence rate to 0.41 (95% confidence interval, 0.19-0.89; P = 0.02).

Conclusions: Although relatively large-scale, simultaneous school closures and social distancing could temporarily suppress seasonal influenza epidemic, they could not impede the cumulative incidence. Large-scale social distancing may contribute to slowing infection spread during seasonal influenza epidemics.