We also showed that lower temperature and lower relative humidity create a higher risk of local transmission of pandemic influenza. However, much lower temperature (e.g., <0°C) did not facilitate local transmission, as indicated by the daily incidence over categorical temperature in Table 2 , and required inclusion of a quadratic variable in the model. The observation of an influence of temperature and relative humidity on pandemic influenza is in accordance with animal experiments on seasonal influenza virus (31) . In addition, recent studies have suggested that absolute humidity could also play an important role in the transmission of pandemic influenza and seasonal variations in influenza epidemics in temperate regions (32) (33) (34) . As expected, population density further facilitated both invasion and local transmission, whereas holiday periods reduced spread (16, 17, 35, 36) . We also used population size as a correction factor instead of population density, which led to similar results (not shown).