Abstract
BACKGROUND: 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. METHODS: 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 χ(2) tests. A season was defined from May 1 to April 30. FINDINGS: 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). INTERPRETATION: 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. FUNDING: Innovation Fund Denmark and Rigshospitalets Forskningsfond.