Background
Some epidemiological studies have suggested an increase in incidence of type 1 diabetes during the COVID-19 pandemic, however the mechanism(s) behind such an increase have yet to be identified. In this study we aimed to evaluate the possible role of the SARS-CoV-2 virus in the reported increase in the rate of type 1 diabetes.

Methods
In this observational cohort study using data from the Finnish Pediatric Diabetes Register (FPDR), we assessed the incidence of type 1 diabetes (number of children with newly diagnosed type 1 diabetes per 100 000 person-years during the pandemic and the reference period) during the first 18 months of the COVID-19 pandemic in children in Finland younger than 15 years old compared with a reference period which included three corresponding pre-pandemic periods also obtained from the FPDR. Children with confirmed monogenic diabetes were excluded. We also compared the phenotype and HLA genotype of the disease between these two cohorts, and analysed the proportion of newly diagnosed people with type 1 diabetes testing positive for SARS-CoV-2 antibodies.

Findings
785 children and adolescents in Finland were diagnosed with type 1 diabetes from March 1, 2020, to Aug 31, 2021. In the reference period, which comprised three similar 18-month terms (from March 1, 2014, to Aug 31, 2015; March 1, 2016, to Aug 31, 2017; and March 1, 2018, to Aug 31, 2019) 2096 children and adolescents were diagnosed. The incidence of type 1 diabetes was 61·0 per 100 000 person-years (95% CI 56·8–65·4) among children younger than 15 years old during the pandemic, which was significantly higher than during the reference period (52·3 per 100 000 person-years; 50·1–54·6). The incidence rate ratio adjusted for age and sex for the COVID-19 pandemic was 1·16 (1·06–1·25; p=0·0006) when compared with the reference period. The children diagnosed during the COVID-19 pandemic had more often diabetic ketoacidosis (p<0·001), had a higher HbA1c (p<0·001), and tested more frequently positive for glutamic acid debarboxylase antibodies at diagnosis (p<0·001) than those diagnosed before the pandemic. There were no significant differences in the distribution of HLA genotypes between the two periods. Only five of those diagnosed during the pandemic (0·9%) of 583 tested positive for infection-induced SARS-CoV-2 antibodies.

Interpretation
Children and adolescents diagnosed with type 1 diabetes during the pandemic had a more severe disease at diagnosis. The observed increase in type 1 diabetes incidence during the first 18 months could be a consequence of lockdown and physical distancing rather than a direct effect of SARS-CoV-2 infection.