Distribution and Risk Factors of 2009 Pandemic Influenza A (H1N1) in Mainland China

Fang, Li-Qun; Wang, Li-Ping; de Vlas, Sake J.; Liang, Song; Tong, Shi-Lu; Li, Yan-Li; Li, Ya-Pin; Qian, Quan; Yang, Hong; Zhou, Mai-Geng; Wang, Xiao-Feng; Richardus, Jan Hendrik; Ma, Jia-Qi; Cao, Wu-Chun

A total of 121,805 cases of pandemic influenza (H1N1-2009), distributed in all 31 provinces in mainland China, were reported from May 9, 2009, to December 31, 2009. There was much variation in the numbers of confirmed cases in different provinces, ranging from 881 to 12,748, with a median of 2,958 cases, and in the incidence of confirmed cases in different provinces, ranging from 3.94 per 100,000 population to 71.72 per 100,000, with a median of 8.41 per 100,000. From the time profile, we found that the number of confirmed cases increased rapidly beginning at the end of August, when a new term began for school students, and peaked by the end of November. The first death caused by pandemic influenza was reported on October 4, 2009. The number of deaths eventually rose to 648 by the end of the year, and peaked in early December (Figure 1 ). The age-and sex-standardized incidence map shows that the epidemic spanned a large geographic area, and the most affected areas were in western China (see Web Figure 1 , which appears on the Journal's website (http://aje.oxfordjournals.org/)). Significant differences in incidence were found among age groups, with incidences peaking in school-age groups (Web Figure 1 ). Boys showed a higher incidence than girls (ages <20 years).