Objective  To analyze seasonal prevalence of influenza virus in Qinghai province and to provide evidences for prevention and control of influenza epidemic in the plateau region.

  Methods  Monthly monitoring data on influenza virus nucleic acid detection in Qinghai, a province located on the Qinghai-Tibet Plateau, from 2009 through 2018 were derived from the Chinese Influenza Surveillance Information System. Epidemiologic description, concentration ratio, circular distribution, and correlation analysis were used to explore temporal distribution of influenza virus prevalence during the period in the region.

  Results  Totally 7 788 influenza virus nucleic acid positive specimens were detected in the region during the 10 years, with higher numbers of positive specimens in 2009, 2014, and 2016 – 2018 than the number in other years. The M values of concentration ratio for the years from 2009 to 2018 were 0.92, 0.85, 0.59, 0.49, 0.85, 0.89, 0.38, 0.90, 0.90, and 0.89, respectively. The parameters of circular distribution of the positive specimens were γ = 0.609 1, \bar \alpha = 340.09 °, S = 57.06, and Z = 2 889.15 (P < 0.05), indicating that during a year the day with the highest influence virus prevalence is December 10 and the period with higher influence virus prevalence is from January 6 to October 14. The M value of concentration ratio was significantly correlated with the gamma value of circular distribution, with a Pearson coefficient of 0.905 (P < 0.001).

  Conclusion  There were annual and seasonal fluctuations in the prevalence of influenza virus and the seasonal fluctuation varied yearly in Qinghai province during 2009 – 2018.