Seasonal variation in the incidence of Type 1 diabetes mellitus during 1983 to 1992 in the countries around the Baltic Sea

Summary Aim To examine seasonal patterns of incidence of Type 1 diabetes mellitus incidence in children aged 0–14 years in Finland, Sweden, Estonia, Latvia and Lithuania during 1983–1992 (1987–1992 for Finland). Methods The study used a method that models incidence data using combinations of sine waves to model seasonal variation around a possible linear trend. Results In Finland, a significant pattern was found for combined sexes and age groups 0–9 and 10–14 years. A significant pattern was also confirmed for 10–14 year‐old boys. In Sweden, the best model with significant pattern was found separately for boys and girls and age groups 0–9 and 10–14 years, however, a significant pattern was confirmed for older girls only. A seasonal pattern in older boys in Finland and girls in Sweden was characterized by two cycles with decreased incidence in June and November–December. The pattern among younger children (0–9 or 5–9 years) had one cycle with a decreased incidence in May–June. In Estonia, a significant pattern was found for the age group 0–14 years and combined sexes. No significant seasonal patterns were found in Latvia and Lithuania. Conclusions The seasonal pattern with two cycles among older children and one cycle only among younger children may indicate different triggers of Type 1 diabetes mellitus for different age groups.