Abstract
IntroductionDuring the coronavirus disease 2019 (COVID-19) pandemic, the circulation of seasonal influenza virus declined globally and remained below previous seasonal levels. We analyzed the results of the epidemiology, antigenic, and genetic characteristics, and antiviral susceptibilities of seasonal influenza viruses isolated from the mainland of China during October 5, 2020 through September 5, 2021, to better assess the risk of influenza during subsequent influenza season in 2021–2022. MethodsPositive rates of influenza virus detection during this period were based on real-time polymerase chain reaction (PCR) detection by the Chinese National Influenza Surveillance Network laboratories, and isolated viruses from influenza positive samples were submitted to the Chinese National Influenza Center. Antigenic analyses for influenza viruses were conducted using the hemagglutination inhibition assay. Next-generation sequencing was used for genetic analyses. Viruses were tested for resistance to antiviral medications using a phenotypic assay and next-generation sequencing.ResultsIn southern China, the influenza positivity rate was elevated especially after March 2021 and was higher than the same period the previous year with the COVID-19 pandemic. In northern China, influenza positive rate peaked at Week 18 in 2021 and has declined since then. Nearly all isolated viruses were B/Victoria lineage viruses during the study period, and 37.3% of these viruses are antigenically similar to the reference viruses representing the vaccine components for the 2020–2021 and 2021–2022 Northern Hemisphere influenza season. All seasonal influenza viruses were susceptible to neuraminidase inhibitors and endonuclease inhibitors.ConclusionsInfluenza activity has gradually increased in the mainland of China in 2021, although the intensity of activity is still lower than before the COVID-19 pandemic. The diversity of circulating influenza types/subtypes decreased, with the vast majority being B/Victoria lineage viruses. The surveillance data from this study suggest that we should strengthen influenza surveillance during the upcoming traditional influenza season. It also provided evidence for vaccine recommendations and prevention and control of influenza and clinical use of antiviral drugs.
Highlights
During the coronavirus disease 2019 (COVID-19) pandemic, the circulation of seasonal influenza virus declined globally and remained below previous seasonal levels
The surveillance data from this study suggest that we should strengthen influenza surveillance during the upcoming traditional influenza season
Influenza is a respiratory illness that infects between 5%–15% of the global population annually during normal seasonal epidemics, and the World Health Organization (WHO) estimates that these infections result in 3–5 million cases of severe illness and about 290,000 to 650,000 respiratory deaths every year [1]
Summary
During the coronavirus disease 2019 (COVID-19) pandemic, the circulation of seasonal influenza virus declined globally and remained below previous seasonal levels. Influenza is a respiratory illness that infects between 5%–15% of the global population annually during normal seasonal epidemics, and the World Health Organization (WHO) estimates that these infections result in 3–5 million cases of severe illness and about 290,000 to 650,000 respiratory deaths every year [1]. Since the March 2020 co-incident with the exponential global spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a dramatic decrease in influenza detection has been observed, despite the testing for influenza continuing at similar levels in many countries [3,4]. The Northern Hemisphere has experienced almost no influenza during the typical timing of the 2020–2021 season, compared to around 18%–23% in 2019–2020, the US and EU reporting only 0.15%–0.20% of samples as positive for influenza [6]. Since September 2020 influenza activity was mostly reported from countries in the tropics and subtropics and some countries in the CCDC Weekly / Vol 3 / No 44
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