Excess hospitalisations associated with influenza, respiratory syncytial virus and SARS-CoV-2 in Singapore from 2015 to 2023

Background: The patterns of circulation and burden of influenza and respiratory syncytial virus (RSV) in Singapore are affected by the COVID-19 pandemic containment measures. These patterns in relation to SARS-CoV-2 in a post-pandemic era are unclear. Methods: Using data from 2015 to 2023, we estimated excess influenza-, RSV- and SARS-CoV-2-associated hospitalisation in Singapore, adjusted for rhinovirus/enterovirus activity in generalised additive models. The data includes pneumonia and influenza (P&I) hospitalisation from a national inpatient database and a community-wide acute respiratory infection (ARI) sentinel surveillance programme, stratified by age groups. Results: Across all age groups, the proportion of hospitalisation associated with influenza, SARS-CoV-2 and RSV was 15.7% (95% CI 6.4%–25.0%), 16.5% (95% CI 10.5%–22.6%) and 6.3% (95% CI 2.2%–12.5%) in 2023. From 2019 to 2023, all-age influenza-associated hospitalisation declined from 129.8 per 100,000 person-years (95% CI 110.9–148.4) to 95.7 per 100,000 person-years (95% CI 38.9–152.2). In contrast, all-age RSV-associated hospitalisation after the pandemic was 38.4 per 100,000 person-years (95% CI 13.1-75.9), similar to pre-pandemic observations. Peak seasonal influenza occurred 3-8 weeks later as compared to the time of pre-pandemic peak influenza activity. Conclusion: The overall burden of influenza has declined after the COVID-19 pandemic and its burden is comparable with SARS-CoV-2. Furthermore, shifts in the timing of peak influenza activity suggest a potential need to review the timing of vaccine recommendations in Singapore.