Etiological surveillance for influenza in Jiuquan, Gansu, 2009-2015

Objective To understand the etiological and epidemiological characteristics of influenza in Jiuquan, Gansu province, from 2009 to 2015, and provide evidence for the prevention and control of influenza. Methods Throat swabs were collected from influenza like illness (ILI) cases in the sentinel hospitals included in national influenza surveillance network for influenza virus nucleic acid detection by real-time RT-PCR and virus isolation, culture and identification were conducted for some positive samples. Results During the seven years, 3332 throat swabs from ILI patients were tested for influenza virus nucleic acid, in which 718 were positive (21.54%), including 277 A/H1N1 virus positive samples (38.58%), 224 A/H3N2 viruspositive samples (31.20%) and 217 B virus positive samples (30.22%), and the annual positive rate of influenza virus were 28.60%, 25.63%, 18.25%, 21.58%, 18.23%, 19.44% and 20.60% respectively from 2009 to 2015. The differences were significant (2=25.51, P=0.000) and the positive rate in age group 5-years was highest (29.07%) among the six age groups, and the positive rates were 6.90%, 15.85%, 29.07, 23.96%, 21.43%and 14.29% respectively in age groups 0-years, 1-years, 5-years, 15-years, 25-years and 60 years, the differences were significant(2=74.51, P=0.000).The positive influenza virus nucleic acid samples were mainly detected from October to March, accounting for 91.50%. A total of 105 influenza virus strains were isolated and identified, including 69 A/H1N1strains, 14 A/H3N2 strains, 10 B/Victoria lineage strains and 12 B/Yamagata lineage strains. Conclusion There was one influenza incidence peak in each year in Jiuquan, which was during winter-spring. The infection was highest in 5-years old children and the predominant influenza virus differed with season and year. Therefore, it is necessary to strengthen influenza vaccination in children aged 5-years before annual influenza season.