Incidence of juvenile idiopathic arthritis in Finland, 2000-2020

Previous epidemiological data of JIA in Finland are from the turn of the millennium. We aimed to determine the recent annual incidence of JIA in several consecutive years in Finland and to explore the differences in incidence between sexes, age groups and regions.

We analysed all children <16 years of age who met the ILAR classification criteria for JIA. Cases from 2000-2020 were identified from two national registers: the Care Register for Health Care of the Finnish Institute for Health and Welfare and the Reimbursement Register containing medication data from the Social Insurance Institution of Finland; cases from 2016-2020 were identified from the Finnish Rheumatology Quality Register.

The incidence of JIA was 31.7 per 100 000 (95% CI 30.2, 33.1), according to the Care Register in 2000-2020 and peaked in 2010-2014. No considerable differences in incidence rates were observed among registers. In all age groups, incidence in girls was predominant compared with boys. The incidence in girls peaked at the ages of 2 years and 14-15 years. Decreasing incidence was observed among boys 0-3 years old during the entire study period, whereas increasing incidence was observed among teenage girls and boys 4-7 years old in 2000-2013.

The incidence of JIA is not only very high with respect to that in other parts of the world but also higher than previously reported in Finland. The incidence varied by region and year but was not higher at the end than the beginning of the study period.