A Case Study on Trends in Acute Respiratory Illnesses and Influenza in Singapore: Pre-, during, and Post-COVID-19 Pandemic.

Background: We investigated trends in acute respiratory illnesses and influenza activity in Singapore throughout the COVID-19 pandemic.

Methods: Publicly available data from the Ministry of Health, Singapore was used to estimate the number of daily acute respiratory illnesses (ARI) and influenza cases between 2019 and Jun 2023. Overseas travel activity was obtained from the Department of Statistics, Singapore. Trends in ARI and influenza activity during different phases of the COVID-19 pandemic with changes in key public health measures were compared.

Results: Pre-pandemic influenza activity exhibited seasonal peaks in Jan and Jul, with daily estimated cases exceeding 40 during these periods. During the early pandemic, influenza cases declined sharply to close to zero and remained at these levels until mid-2022, even after public health measures were eased. ARI cases followed a similar initial decline but gradually returned to pre-pandemic levels by 2023, with peaks occurring outside the usual seasonal pattern. These trends corresponded with phases of public health measures, including mask-wearing and travel restrictions.

Conclusion: Several hypotheses for these contrasting trends were explored, including increased public health awareness, higher influenza vaccination rates, and potential virus-virus interactions.