Epidemiological and etiological surveillance for influenza in Qingyang, Gansu, 2018−2022

Objective To understand the epidemiological and etiological characteristics of influenza in Qingyang, Gansu province, from 2018 to 2022 and provide data support for the effective prevention and control of influenza in the future.

Methods The etiological surveillance data of influenza-like illness (ILI) in local sentinel hospitals were obtained from Chinese Influenza Surveillance Information System, a subsystem of Chinese Information System for Disease Control and Prevention. ILI case samples were collected from the sentinel hospitals for the extraction of influenza virus nucleic acid, and real-time fluorescence quantitative PCR was used for typing. Madin-Darby Canine Kidney cell culture and chicken embryo culture were performed for the nucleic acid positive samples to isolate and identify the viruses. The data were processed by using Excel 2007, and statistical analysis was performed by using software SPSS 22.0.

Results From 2018 to 2022, a total of 6 473 ILI cases were recorded in Qingyang. The annual average ILI case percentage was 1.34%. The ILI case percentage showed a decreasing trend (trend χ²=1165.377, P=0.00). The ILI cases were mainly distributed in children aged 0–4 years (60.68%), followed by those aged 5–14 years (26.30%). From 2020 to 2022, the overall average ILI case percentage was 0.85%, showing a clear decreasing trend, and the ILI case percentage in all age groups decreased too, but the decrease was more obvious in older age group than in younger age group. The results of epidemiological and etiological surveillance demonstrated that the incidence of influenza had seasonal characteristics, with an annual incidence peak in winter and spring. From 2018 to 2022, a total of 3 574 ILI case samples were detected, in which 621 were positive for the influenza viruses, with the positive rate of 17.38%. The differences in influenza positive rate among different years were significant (χ²=55.778, P<0.001). Among the positive samples of different types, the positive samples of influenza A and B viruses accounted for 46.38% and 52.66% respectively, while the co-positive samples accounted for 0.97%. Two positive samples of Eurasian avian-like H1N1 swine influenza virus were detected in routine surveillance in 2021. The incidence intensity of influenza B was generally higher than that of influenza A, but influenza A virus and influenza B virus spread together or alternatively in different years.

Conclusion The epidemiological and etiological characteristics of ILI in Qingyang indicated that the surveillance for influenza viruses should be further strengthened. It is crucial to improve the surveillance and detection of influenza viruses to understand the virus variation for the development of effective influenza prevention and control strategies as well as the accurate prediction and control of seasonal influenza epidemic.