Background: Influenza is a common cause of outpatient visits in childhood. Influenza vaccine has been widely used for children ≥ 6 months of age in developed countries based on accumulating evidence on the substantial socioeconomic burden of influenza in children. However, the data of childhood influenza in China are scarce.
Methods: We carried out a prospective surveillance of influenza among children visiting outpatient clinic for influenza-like illness between June 2009 and May 2011. Respiratory samples were obtained from the enrolled cases and the demographic and clinical data were collected.
Results: Of 2356 enrolled cases, influenza was virologically confirmed in 608 (25.81%) otherwise healthy children. Four hundred sixty-eight (76.97%) were influenza A positive (A/H3N2:236, pandemic A/H1N1:219, seasonal A/H1N1:4, A/H3N2 coinfected with pandemic A/H1N1:3, untyped:6) and 142 (23.35%) were influenza B positive (2 coinfected with pandemic A/H1N1). The outbreaks of A/H3N2, pandemic A/H1N1 and influenza B took place in fall, winter and spring, alternatively. More institutionalized children were affected when the outbreaks overlapped with school opening. Among 603 children with a single influenza virus infection, 289 (47.93%) were <3 years old and 283 (46.93%) had close contact with persons who had fever or respiratory infection, of whom, 192 (67.84%) contacts were family members.
Conclusion: The annual outbreak of influenza led to a high rate of outpatient visits among children in Shanghai. Outbreaks occurred in younger and institutionalized children. Household members were the main disseminators to spread influenza among children.