Epidemiology of Invasive Group A Streptococcal Infection in Children and its Association With Community Circulation of Scarlet Fever and Respiratory Viral Infections.

BACKGROUND: Streptococcus pyogenes (group A Streptococcus, GAS) is a common pediatric pathogen causing a wide range of diseases with significant morbidity and mortality. GAS infections exhibit strong seasonal patterns, and the invasive forms (iGAS) have historically been associated with influenza. Since September 2022, a global outbreak of both iGAS and noninvasive GAS infections has been reported. METHODS: This is a retrospective observational multicentric study of children (<15 years) with iGAS in Catalonia (Spain) from week 40 of 2022 to week 24 of 2023. We collected clinical, epidemiologic and microbiologic data on iGAS, scarlet fever and acute respiratory infection diagnoses between 2013 and the study period. Seasonal comparisons were assessed using Kruskal-Wallis and Wilcoxon tests, and correlations between weekly time series were evaluated using Spearman coefficients. RESULTS: Among 135 pediatric iGAS cases, the median age was 3.7 years (interquartile range, 1.6-5.9), mostly healthy. Fever was the most common symptom (92.6%), and pneumonia was the most prevalent diagnosis (40.7%). Viral coinfection occurred in 59.3% of the cases, with influenza A/B being the most prevalent. One-third (35.6%) required intensive care; the case fatality rate was 3.0%. We found a significant correlation between pediatric iGAS infections and influenza A/B circulation (ρ = 0.85) in the context of significantly higher scarlet fever incidence compared with pandemic and pre-pandemic seasons. Weaker correlations were identified between iGAS infections and the other acute viral infections studied. CONCLUSIONS: Pediatric iGAS during the 2022-2023 outbreak was strongly associated with influenza activity, suggesting that influenza might contribute to the epidemiology of pediatric iGAS in a context of increased GAS circulation.