Shifts in seasonal influenza patterns in Australia during and after COVID-19: A comprehensive analysis

Background: During the COVID-19 pandemic, seasonal influenza virus circulation was heavily suppressed worldwide. In Australia, since the virus re-emerged in 2022, shifts in seasonal influenza patterns have been observed. Both the 2022 and 2023 seasons started earlier than pre-pandemic norms and were categorised as moderate to severe, highlighting the renewed importance of prevention strategies for seasonal influenza.

Methods: We analysed influenza notification data from the Australian National Notifiable Diseases Surveillance System (2012-2022) and virological surveillance data from the FluNet database (2012-2023). Using generalised additive models, we compared predicted weekly influenza case counts during 2020-2022 with observed counts. Epidemic weeks were detected using a negative binomial threshold, and epidemic onset was estimated with a Bayesian Poisson count detection algorithm. Trends in epidemic magnitude and onset timing across influenza virus types and subtypes were compared for pre-, during, and post-COVID-19 periods.

Results: Seasonal influenza activity was nearly absent in 2020 and 2021 but rebounded significantly in 2022 and 2023. Epidemic detection confirmed suppressed seasonal influenza circulation during the pandemic. While influenza A subtypes returned to pre-pandemic onset timings in 2022, influenza B exhibited a significantly delayed onset. The 2022 and 2023 seasons were moderate to severe, with earlier-than-average season starts, underscoring the ongoing changes in influenza dynamics post-pandemic.

Conclusions: This study provided a detailed analysis of the disruptions and subsequent shifts in seasonal influenza patterns in Australia during and after the COVID-19 pandemic. The rapid resurgence of influenza activity in 2022 and 2023, combined with altered onset timings, highlights the importance of ongoing surveillance and adaptive forecasting models to address the evolving complexity of influenza epidemiology in the post-pandemic era.