Temporal Trends in the Incidence and Prevalence of Multiple Sclerosis in Sweden, 2001-2021

Understanding secular trends in demographic patterns of multiple sclerosis (MS) may help elucidate its etiopathogenesis and is also important for future resource allocation and planning healthcare strategies. Though several incidence and prevalence studies exist, nationwide studies utilizing contemporary data collected over decades remain rare. The objective of this study was to determine changes in incidence and prevalence of MS in Sweden from 2001 to 2021.

Leveraging data from multiple linked population-based registries, we utilized two case definitions: one administrative (based on International Classification of Disease codes from inpatient and outpatient visits, 2006-2021) and one clinical (based on the Swedish MS registry, 2001-2021). We estimated the overall and annual incidence and point prevalence using both definitions and age-standardized to the world standard population. Temporal trends in incidence and prevalence were analyzed using joinpoint and binomial regression, respectively.

In total, 13,055 incident and 28,107 prevalent MS cases were identified in the administrative data. The overall age-standardized incidence rate was 8.37 (95% confidence interval [CI] 8.22-8.52) cases per 100,000 person-years. Incidence rates were stable until 2012, then decreased until 2021 (annual percent change -2.51%; 95% CI: -5.04% to -1.69%), which translated into a decrease in number of new MS cases from 796 in 2006 to 701 in 2021, or -12.0%. In the clinical data, the overall age-standardized incidence was 7.87 (95% CI: 7.74-7.99) cases per 100,000 person-years. Incidence increased between 2001 and 2014 with an annual percent change of 2.00% (95% CI: 1.40%-2.84%) and thereafter decreased through 2021 with an annual percent change of -2.84% (95% CI: -5.06 to -1.49%). The age-standardized prevalence of MS in the administrative cohort increased from 118 cases per 100,000 (95% CI: 116 to 120) in 2006 to 167 cases per 100,000 (95% CI: 164 to 169) in 2021, i.e., an increase of 41.7%. Similarly, in the clinical cohort, prevalence increased from 71 (95% CI: 69 to 72) to 154 (95% CI: 151 to 156) cases per 100,000 persons from 2001 to 2021, representing an increase of 117.9% Conclusion: Our findings demonstrate a decreasing incidence of MS in the last decade, while prevalence continuously increased. Although confounding by changes in data density cannot be excluded, this may reflect changes in environmental exposures such as smoking, as well as contemporary treatment practices and a secular trend toward milder disease with reduced mortality.