[Influenza-like illness outbreaks in China during 2017-2018 surveillance season]

Objective: To analyze epidemiological characteristics of influenza-like illness outbreaks in mainland China during 2017-2018 surveillance season, and to provide scientific evidence for developing influenza prevention and control strategies. Methods: We collected the data on reported influenza outbreaks in 2017-2018 surveillance season from China Influenza Surveillance Information System and China Public Health Emergency Management Information System and analyzed the data of laboratory-confirmed influenza-like illness outbreaks by descriptive epidemiological methods. Results: During the surveillance season, a total of 2 398 influenza-like illness outbreaks (with 10 or more incidences in an outbreak) in mainland China were reported, involving 87 084 patients, of which 2 323 were influenza outbreaks, involving 85 531 patients. The reported influenza-like illness outbreaks occurred most frequently from November 2017 to January 2018 in both the southern and northern regions and the highest peaks were in December 2017. During the period 1 850 influenza-like illness outbreaks (77.15%) were reported in the southern region, and 548 influenza-like illness outbreaks (22.85%) were reported in the northern region. The most of the outbreaks occurred in primary, secondary schools and nursery care schools, with a total of 2 210 reports (92.16%). And the majority of the outbreaks involved 10-29 incident cases. The dominant isolated virus strains for the outbreaks were influenza B (1 505 outbreaks, 62.76% of all the outbreaks). Conclusion: Seasonality of influenza outbreaks were observed in mainland China during 2017-2018 surveillance season and the reported influenza outbreaks were most frequently occurred in autumn-winter season and in southern China. Primary, secondary schools and nursery care schools are high-risk places for outbreaks, and the dominant isolated virus strains for the outbreaks were influenza B.