Web Figure 2 shows the trend of the spatial spread of pandemic influenza over time in mainland China and indicates that the epidemic areas during the first 120 days after May 9, 2009, were limited to the circumferences of cities with international airports, such as Beijing, Shanghai, Guangzhou, Shenzhen, Chengdu, and Changchun. Thereafter, it spread to the rest of mainland China, roughly from southeast to northwest. The largest-scale spread took place 150-180 days after the first case (Web Figure 2) . Figure 2 shows the large variation in the temporal patterns of pandemic influenza for the 5 selected cities/provinces, but there was a marked drop in incidence during the first week of October for all locations.