Age‑period‑cohort and Joinpoint analysis of hand, foot and mouth disease incidence in Dalian, 2009–2024, with special reference to the change in proportion of age group during 2023–2024

To analyze age distribution pattern, evolution trend and variations over time of hand foot and mouth disease(HFMD) incidence rate, in Dalian city, from 2009 to 2024. HFMD case data in Dalian from 2009 to 2024 were collected. Joinpoint regression was used to analyze trend in incidence rates as age increases among children aged 0–14 years across three periods, and an age-period-cohort (APC) model was constructed to assess effects of age, period, and birth cohort. 1)From 2009 to 2024, there were 110117 HFMD cases reported in Dalian. (2)Age group 0–4 years consistently had highest annual incidence of HFMD, since 2023, proportion of cases in age group 10–14 years has increased. (3)Joinpoint regression analysis showed that during 2009–2019, HFMD incidence decreased significantly with increasing age in most age-sex groups.Except for 2023–2024 for female, age-specific trends was significant heterogeneity. (4) APC analysis showed that net drift test was statistically significant, with an estimated net drift of -6.848%(95%CI: -13.097%~-0.15%)and − 8.739%༈95%CI༚-14.208%~-2.921%༉respectively in males and females. The Cohort RR curve showed that, rate ratio of 2016–2020 birth cohort were 0.182 (95% CI:0.098–0.337) and 0.17 (95% CI:0.1–0.289)) respectively in males and females.For male, The local drift values in the age group 0–4 years,5–9 years and 10–14 years were − 15.681%(95% CI:-20.736%--10.304%༉, -5.401%༈95% CI:-12.158%-1.874%༉ and 1.335%༈95% CI:-15.137%-21.006%༉per year of age respectively, that for female were − 16.232% ༈95% CI:-20.569%--11.659%༉, -7.377%༈95% CI:-13.378%--0.961%༉ and − 2.037%༈95% CI:-6.179%-14.491%༉per year of age respectively. Since 2023, the proportion of HFMD cases in age group 10–14 years increased, likely driven more by declining cases in 0–4 years children than by increased in risk among age groups 10–14 years. Significant age-period‑cohort effects were observed in both sexes, with the 2016–2020 birth cohort having the lowest risk, but these findings are exploratory due to APC analysis limitations and require further validation.