Epidemiological characteristics of influenza in Shenzhen, Guangdong province, 2014−2021

  Objective  To understand the epidemiological characteristics of influenza in Shenzhen and provide evidence for prevention and control of influenza.

  Methods  The surveillance data of influenza like illness (ILI) and etiological surveillance data were collected in Shenzhen from 2014 to 2021 for a statistical analysis.

  Results  A total of 1363262 ILI cases were reported in Shenzhen from 2014 to 2021 with ILI rate of 4.09%. The proportion of ILI cases was highest in 2021 (4.85%) and lowest in 2017 (3.60%). A total of 38010 swabs were collected from ILI cases, from which 3438 influenza virus strains were isolated and identified with a positive rate of 9.04%, including 1053 strains of influenza A (H1N1) virus (30.63%), 1038 strains of influenza A (H3N2) virus (30.19%), 741 strains of influenza B virus Victoria-lineage (21.55%) and 606 strains of influenza B virus Yamagata-lineage (17.63%). There were winter-spring and summer incidence peaks in 2014, 2015 and 2019. There were winter-spring and autumn incidence peaks in 2016. There was spring-summer incidence peak in 2017. There was winter-spring incidence peak in 2018. The influenza activity level was low during 2020−2021 except an autumn influenza epidemic in 2021. The positive rate of influenza virus detection was highest in spring (12.47%), followed by winter (11.73%), summer (8.61%) and autumn (3.12%).

  Conclusion  Four types of influenza viruses circulated alternatively as the predominant pathogen in Shenzhen from 2014 to 2019. Influenza activity mainly occurred in spring, winter and summer, rarely in autumn. The incidence intensity of influenza A was stronger than that of influenza B. The influenza activity level was low during 2020–2021.