Changes in respiratory virus seasonality across the COVID-19 pandemic: a multicentre comparative study in Leicester, Rotterdam, Hong Kong, and Christchurch.

OBJECTIVES: To describe respiratory virus seasonality before, during, and after coronavirus disease 2019 (COVID-19) using routine hospital diagnostic laboratory data contributed through the International Network for the Sequencing of Respiratory Viruses (INSPIRE). METHODS: We analysed monthly test data from four diagnostic laboratories in Leicester, UK; Rotterdam, the Netherlands; Hong Kong, China; and Christchurch, New Zealand, during 2018-2023. Viruses included influenza A virus, influenza B virus, respiratory syncytial virus, parainfluenza viruses 1-4, human metapneumovirus, adenovirus, and enterovirus/rhinovirus. Monthly test positivity was compared across pre-COVID-19 (2018-2019), COVID-19 disruption (2020-2021), and post-COVID-19 re-emergence (2022-2023) periods. RESULTS: The dataset comprised 3,164,720 tests performed and 84,300 positive detections. During 2020-2021, influenza activity was markedly suppressed: influenza A virus positivity decreased from 7.3% to 1.2%, and influenza B virus positivity from 3.0% to 0.1%. Respiratory syncytial virus showed a smaller overall reduction, whereas enterovirus/rhinovirus remained detectable in all four sites. In 2022-2023, respiratory viruses re-emerged heterogeneously; peak months differed by at least two months between the pre- and post-COVID-19 periods in 21 of 40 site-virus combinations. CONCLUSIONS: Respiratory virus seasonality was substantially altered during and after COVID-19. Post-COVID-19 re-emergence varied by virus and site, supporting continued multi-pathogen surveillance.