Changes in respiratory virus seasonality across the COVID-19 pandemic: a multicenter 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 analyzed 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 2 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.