Abstract: Objective To investigate the seasonal characteristics of hand, foot, and mouth disease (HFMD) in Longgang District, Shenzhen, from 2014 to 2024, and to analyze the influence of the COVID-19 pandemic on these seasonal patterns, with the aim of providing a scientific basis for the early warning and prevention and control of HFMD. Methods Seasonal and Trend decomposition using Loess (STL), the seasonal index method, concentration analysis, and circular distribution were employed to analyze the seasonal characteristics of HFMD in Longgang District from 2014 to 2024. Results The annual incidence rate of HFMD in Longgang District between 2014 and 2024 ranged from 61.67/100 000 to 709.45/100 000. There was no statistically significant difference in the incidence of HFMD before and after the COVID-19 pandemic (χ²=1.36, P>0.05). Both the STL and seasonal index methods revealed a bimodal epidemic pattern for HFMD in Longgang District, with peaks occurring from May to July and from September to October. Prior to the COVID-19 pandemic, a bimodal distribution was predominant. However, during the pandemic (with the exception of 2021) and in the post-pandemic period, a unimodal distribution was primarily observed, and its intensity was greater than in previous years. Concentration analysis indicated a certain degree of seasonality in the incidence of HFMD from 2014 to 2024 (concentration value M=0.461). Upon exclusion of data from the COVID-19 period (2020-2022), a stronger seasonality was observed (concentration value M=0.501). Circular distribution analysis yielded r=0.08 and α=210.91°(Z=638.93>Z_0.001,P<0.001), with the epidemic peak day identified as June 15th and the peak period extending from April 11th to August 20th. Conclusion The incidence of HFMD in Longgang District, Shenzhen, exhibited distinct seasonal characteristics from 2014 to 2024, with a bimodal epidemic pattern in May-July and September-October. Nevertheless, the seasonal pattern shifted to a unimodal distribution following the COVID-19 pandemic. It is imperative to monitor this change in characteristics and its duration to facilitate the timely adjustment of corresponding prevention and control measures.

Key words: Hand, foot, and mouth disease, Seasonal and Trend decomposition using Loess (STL), Seasonal index method, Concentration analysis, Circular distribution