Multisystem inflammatory syndrome in children (MIS-C) is a severe manifestation of SARS-CoV-2 in children and adolescents.¹ We aimed to estimate the risk of MIS-C after SARS-CoV-2 infection in vaccinated and unvaccinated individuals during the Omicron wave. Further, we aimed to compare the risk and clinical characteristics of MIS-C with the pre-Omicron waves.

Mycoplasma pneumoniae infections resurged globally in 2023–2024 after a three-year decline during the COVID-19 pandemic. We explored the incidence and severity of M pneumoniae infections in children and adolescents before, during, and after the pandemic. This nationwide, population-based cohort study included all Danish children and adolescents aged 0–17 years with a positive M pneumoniae PCR test from May 1, 2016, to April 30, 2024. We obtained clinical details for patients hospitalised for 24 h or more. Risk ratios for infections, hospitalisations, and disease manifestations in 2023–2024 versus pre-COVID-19 seasons were calculated using Fisher’s exact and Pearson’s χ² tests. A season was defined from May 1 to April 30. Among the Danish population of 1,152,000 children and adolescents, 14,241 with a positive PCR test for M pneumoniae were included. In 2023–2024, children and adolescents with a positive PCR rose 2.9-fold (95% CI 2.8–3.1; p < 0.0001) compared to the pre-COVID-19 seasons, and hospitalisations rose 2.6-fold (95% CI 2.0–3.3; p < 0.0001). M pneumoniae-induced rash and mucositis increased 5.3-fold (95% CI 1.8–15.3; p = 0.0007). In 2023–2024 compared to the pre-COVID-19 seasons, there was no difference in the proportion of hospitalisation (360 [4%] of 8165 versus 230 [4%] of 6009; p = 0.09), the median duration of hospital stay (3 days [IQR 2–5] versus 3 days [IQR 2–5]; p = 0.84), or paediatric intensive care unit admission (14 [4%] of 360 versus 9 [4%] of 230 p = 1.00). In Denmark, M pneumoniae infections and hospitalisations increased three-fold in 2023–2024 compared with the pre-COVID-19 seasons, indicating an immunity debt caused by the decline in M pneumoniae during the COVID-19 pandemic. While the severity of M pneumoniae infections did not change in 2023–2024, the five-fold increase in M pneumoniae-induced rash and mucositis in children and adolescents highlights M pneumoniae as an important pathogen causing mucocutaneous eruptions. Innovation Fund Denmark and Rigshospitalets Forskningsfond.