Etiological surveillance for influenza-like illness in Hunan, 2017−2022

Objective To understand the performance of the etiological surveillance for influenza-like illness (ILI) in Hunan province from 2017 to 2022, and provide evidence for influenza prevention and control in Hunan.

Methods Throat swabs of ILI cases from 23 local sentinel hospitals were collected every week, and real-time reverse transcription-polymerase chain reaction was used to detect and identify influenza viruses in the specimens. The positive samples were used for virus isolation by cell culture, and the test results were described. The differences in positive rate of influenza virus detected among different years, gender and age groups were analyzed by χ² test.

Results From 2017 to 2022, a total of 162 773 samples from ILI cases were detected, in which 20 885 were positive for influenza virus, with a positive rate of 12.83%. Four types of influenza viruses were detected, i.e. influenza A (H1N1) virus (19.42%), influenza A (H3N2) virus (33.77%), influenza B virus Victoria lineage (37.14%) and influenza B virus Yamagata lineage (9.50%). In the 6 years, the positive rate was lowest in 2020 (3.76%) and highest in 2019 (21.50%). The incidence peak of influenza mainly occurred in winter and occasionally in summer and spring. The positive rates of influenza virus detected in men and women were 12.81% and 12.85%, respectively. In different age groups, the positive rate was highest in age group 5-15 years (21.69%) and lowest in age group ≥60-years (5.83%). The positive rate in different prefectures ranged from 6.86% to 17.47%.

Conclusion The incidence of influenza showed seasonal characteristics in Hunan from 2017 to 2022. The predominant influenza virus varied with year. It is necessary to strengthen influenza surveillance for the better assessment of incidence trend of influenza.