Temporal association and seasonal variation of respiratory viruses and invasive group A streptococcal infections in western Sweden: A 15-year analysis across pre- and post-pandemic periods

OBJECTIVES: To examine the seasonal variation of invasive group A streptococcal infections (iGAS) in relation to respiratory viral circulation, with special reference to pandemic restrictions, and to assess whether increases in viral activity may explain recent increases in iGAS. METHODS: We performed a time-series analysis using a multivariate negative binomial regression model, with weekly counts of respiratory virus detections as the independent variable and weekly counts of iGAS cases as the dependent variable, covering the period from January 2010 to December 2024. RESULTS: We found a significant association between influenza and iGAS, with increases in influenza activity followed by an increase in iGAS cases. The association was strongest at a 2-week time lag. Influenza A (incidence rate ratio 1.0050, P