Despite the rigorous efforts before the global eradication, later progress on this issue was unfortunately subtle. Upham once revisited Rogers's dataset from India, anthropologically discussing potential reasons why the American Southwest was less infested by smallpox [29] . A time series technique was applied to historical data in Finland and England [30] [31] [32] , showing that periodicity is mainly regulated by the susceptible fraction of a population in question [33] . However, despite the analyses on the impact of vaccination and migration on periodicity, seasonal patterns of transmission have not been explicitly studied, mainly because of a lack of data precision. In a historical study examining smallpox in England from the 16th to 17th centuries, the time referred to as the "little ice age," it has been documented that longterm climatic changes did little to the smallpox transmission [34] , but this conclusion was drawn without quantitatively and explicitly analyzing the data. Instead, the quality of time series data and its impact on seasonality were discussed in relation to social backgrounds of smallpox control [35, 36] , but again no rigorous statistical analyses were made using observed data.