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.