Smallpox and Season: Reanalysis of Historical Data

Seasonal variation in smallpox transmission is one of the most pressing ecological questions and is relevant to bioterrorism preparedness. The present study reanalyzed 7 historical datasets which recorded monthly cases or deaths. In addition to time series analyses of reported data, an estimation and spectral analysis of the effective reproduction number at calendar time t , R ( t ), were made. Meteorological variables were extracted from a report in India from 1890–1921 and compared with smallpox mortality as well as R ( t ). Annual cycles of smallpox transmission were clearly shown not only in monthly reports but also in the estimates of R ( t ). Even short-term epidemic data clearly exhibited an annual peak every January. Both mortality and R ( t ) revealed significant negative association ( P < .01) and correlation ( P < .01), respectively, with humidity. These findings suggest that smallpox transmission greatly varies with season and is most likely enhanced by dry weather.