The re-emergence of influenza following the COVID-19 pandemic in Victoria, Australia (preprint)

ABSTRACT

Background: COVID-19 pandemic mitigation measures, including travel restrictions, effectively limited global circulation of influenza viruses. In Australia, travel bans for non-residents and quarantine requirements for returned travellers were eased in November 2021, providing pathways for influenza viruses to be re-introduced. Methods: From 1 November 2021 to 30 April 2022 we conducted an epidemiological study to investigate the re-establishment of influenza in Victoria, Australia. We analyzed case notification data from the Victorian Department of Health to describe case demographics, interviewed the first 200 cases to establish probable routes of virus reintroduction, and examined phylogenetic and antigenic data to understand virus diversity and susceptibility to current vaccines. Results: Overall, 1598 notifications and 1064 positive specimens were analyzed. The majority of cases occurred in the 15-34 year age group. Case interviews revealed a higher incidence of international travel exposure during the first month of case detections and high levels of transmission in university residential colleges associated with the return to campus. Influenza A(H3N2) was the dominant subtype, with a single lineage predominating despite multiple importations. Conclusions: Enhanced testing for respiratory viruses during the COVID-19 pandemic provided a more complete picture of influenza virus transmission compared to previous seasons. Returned international travellers were important drivers of the re-emergence of influenza, as were young adults, a group whose role has previously been under-recognised in the establishment of seasonal influenza epidemics. Targeting interventions, including vaccination, to these groups could reduce influenza transmission in the future.