Continued High Levels of Invasive Streptococcus pyogenes Infections in Belgian Children Under 5 Years: Exploring Different Contributing Factors

BACKGROUND: Many countries saw an increase in invasive infections caused by Streptococcus pyogenes (iGAS) late 2022 to early 2023. Several hypotheses have been put forward to explain this increase, but thus far, reports have been limited to the immediate post-pandemic period, and the relative contribution of each of those factors remains unknown. METHODS: We describe the epidemiology of iGAS in Belgium in children below 5 years of age from January 2014 until December 2024 based on laboratory data from the National Reference Centre. Using a cointegration analysis after removing seasonality, we formally assess the influence of co-circulating viral diseases varicella, respiratory syncytial virus and influenza on iGAS case numbers. RESULTS: After very low levels during the pandemic years, a marked increase started in June 2022, reaching an all-time high in April 2023 before decreasing but still remaining at high levels in early 2024. Reduced population immunity alone cannot explain this pattern. Emm 12, emm 1 and emm 3 genotypes succeeded each other as the predominant genotype, indicating a complex interplay of transmission and virulence factors. Levels of varicella infections, but not respiratory syncytial virus or influenza, are closely linked to the increase in iGAS levels. CONCLUSION: Levels of iGAS infections remained high up to 2 years after lifting nonpharmaceutical interventions, and this increase is undoubtedly multifactorial. Continued high-quality surveillance is required to follow up the situation.