Dual Seasonal Patterns for Influenza, China

To the Editor: Since 2000, the People's Republic of China has had a nationwide surveillance network for infl uenza, which as of 2005 has been reported on the Chinese Center for Disease Control and Prevention website (www.cnic.org.cn/ch/). This surveillance has shown a remarkable dual pattern of seasonal infl uenza on mainland China. Whereas a regular winter pattern is noted for northern China (similar to that in most parts of the Northern Hemisphere), the pattern in southern China differs. In southern China, infl uenza is prevalent throughout the year; it has a clear peak in the summer and a less pronounced peak in the winter. Because this dual seasonal pattern of infl uenza has not been reported outside China and is relevant to pandemic (H1N1) 2009, we describe surveillance data for rates of consultation for infl uenza-like illness (ILI) and infl uenza subtypes in patients with ILI. We emphasize the spread of infl uenza from southern to northern China. Before it was extended in June 2009, the National Infl uenza Surveillance Network had been composed of 63 infl uenza laboratories and 197 sen-tinel hospitals across 31 provinces of mainland China. In 13 of 16 northern provinces, surveillance began from the week including October 1 and ended in the week including March 31 of the following year. In the 3 northern provincial areas of Liaoning, Tianjin, and Gansu and in all southern provinces, surveillance was conducted throughout the year. Data consisted of information about ILI cases and virus sub-types. The sentinel hospitals defi ned ILI cases according to World Health Organization criteria: sudden onset of fever >38°C, cough or sore throat, and absence of other diagnoses (1). The number of ILI cases and the total number of outpatients at the sites (ILI consultation rate) were recorded each day and reported to the National Infl u-enza Surveillance Information System each week. Sentinel hospitals were required to collect 5–15 nasopharyngeal swabs each week from ILI patients who had not taken antiviral drugs and who had fever (>38°C) for no longer than 3 days. The swabs were sent to the corresponding infl uenza laboratories for virus isolation and identifi cation; results were reported to the National Infl uenza Surveillance Information System within 24 hours. From the National Infl uenza Surveillance Network, a database of surveillance information from April 2006 to March 2009 was established. For infl uenza surveillance purposes, mainland China was divided into northern …