Seasonal Variation in Type 1 Diabetes Incidence in Poland: Exploring the Impact of Viral Infections, Including COVID-19.

Seasonal variation in type 1 diabetes (T1D) incidence has long been a focus of epidemiological research, with viral infections among the proposed contributing factors. Our aim was to examine the seasonal pattern of T1D onset in Poland and to assess how viral infections-including COVID-19-may influence this seasonality. We analyzed data from 2381 children with newly diagnosed T1D admitted to two pediatric diabetes centers in the Masovian Voivodeship between 2015 and 2023 and compared them with epidemiological data on COVID-19 and influenza cases during the same period. Our analysis revealed a 30% increase in T1D cases over the study period, with a pronounced seasonal pattern: the highest number of diagnoses occurred in February and the lowest was noted in June. Children under 4 years of age exhibited a distinct pattern with a peak in October, suggesting age-specific differences in T1D pathogenesis. Overall, T1D onset was more frequent in autumn-winter than in spring-summer, with 1294 (54%) vs. 1087 (46%) cases, respectively (  < 0.0001). The influence of COVID-19 on T1D incidence was limited to the first wave of the pandemic. During this period, a strong association was observed ( = 0.96,   < 0.001), whereas no correlation was found during the second wave ( = 0.086, = 0.87). The seasonality of T1D diagnoses closely correlated with that of influenza infections ( = 0.79, = 0.002). However, the overall trends differed, suggesting that other viruses with similar transmission patterns may contribute to the seasonality of T1D onset. These findings underline the complex interplay between viral infections and T1D seasonality and suggest that public health strategies aimed at mitigating severe viral infections, including vaccination, warrant further investigation for their potential role in modulating T1D onset in susceptible individuals.