Seasonal Dynamics of Pediatric Herpangina and Hand, Foot, and Mouth Disease in Catalonia, Spain (2014-2025): A Primary Care-based Epidemiologic Study.

BACKGROUND: Herpangina and hand, foot, and mouth disease (HFMD) are common pediatric enterovirus syndromes, but long-term evidence from European primary care remains scarce. METHODS: Primary care electronic health records from the public healthcare system in Catalonia, Spain, were retrospectively analyzed for the academic years 2014-2015 to 2024-2025 using International Classification of Diseases, 10th revision codes. Incidence trends, age and sex patterns, seasonal peak timing, and peak-to-peak dependence were assessed in children younger than 5 years. RESULTS: A total of 137,460 diagnoses were identified, including 54,060 herpangina and 83,400 HFMD. Both syndromes showed recurrent bimodal seasonality aligned with school holidays. Peak magnitudes were negatively correlated at both 6-month and 1-year lags, consistent with susceptible depletion and replenishment. Before the pandemic, spring herpangina peaks shifted about 1.5 weeks later for each 1-week delay in Easter. Incidence declined sharply in 2020 and rebounded from autumn 2021, particularly for HFMD. Most diagnoses occurred in children aged 1-2 years, with a transient post-pandemic shift toward ages 3-4 years in the first post-pandemic season. Age-specific curves were highly synchronized, with no pediatric age group consistently preceding the others. CONCLUSIONS: This large primary care series, among the very few long-term European studies of herpangina and HFMD, supports structured seasonal dynamics shaped by school-calendar timing and susceptible dynamics over successive seasons. Long-term primary care surveillance may help interpret atypical transmission patterns and support pediatric preparedness when virologic surveillance is limited.