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

In the initial phase of the epidemic, the Chinese government took measures to prevent and control the spread of the novel influenza virus, declaring it a notifiable infectious disease in order to strengthen national surveillance and find newly confirmed cases quickly. In addition, quarantine measures were implemented at the international airports (e.g., Beijing, Shanghai, Guangzhou, and Fuzhou) to identify and isolate prob-able cases and close contacts in order to decrease the risk of local transmission caused by imported cases. It is possible that these measures were effective in achieving a slower pace of spread in the early stages of the epidemic, but the current evidence is inconclusive (22) . However, following the development of the global pandemic and the beginning of the new school term, a rapid spread of the epidemic occurred in mainland China, and local outbreaks increased at the end of August 2009. A reduction in incidence was observed during the first week of October, when there was an 8-day public holiday during the National Days from October 1 to October 8. This drop in incidence was largely due to a lower tendency for patients to visit medical facilities at that time, together with the fact that many hospitals had reduced the open hours of their outpatient clinics during the holidays. This is clearly visible in Figure 1 , where we see the drop beginning on September 28, 3 days (i.e., the average duration between onset and seeing a physician) before the start of the holiday period. Thus, the holiday period led to a reduction in the number of people being diagnosed with pandemic influenza. Apparently, most undiagnosed patients recovered in the following days, because there was no marked compensation visible in the days after the holiday period. Additionally, there Abbreviations: CI, confidence interval; NS, not significant. a Results were adjusted for school summer vacation and public holidays, population density, and the density of medical facilities. b For all continuous variables, categorical results are also reported to allow inspection of the data and assessment of whether or not the assumption regarding continuous variables was justified. may have been some reduced transmission because of school closure, as was observed in Japan, where transmission was substantially reduced during school closure (23) . In addition, the temporal death curve could reflect the massive rise in confirmed cases with approximately 1 week's delay following the peak of confirmed cases at the end of November.