Incidence and prevalence of paediatric-onset multiple sclerosis in two Canadian provinces: a population-based study representing over half of Canada's population.

BACKGROUND: Population-based studies estimating the epidemiology of paediatric-onset multiple sclerosis (PoMS) are scarce. METHODS: We accessed population-based health administrative data from two provinces in Canada, Ontario and British Columbia (BC). Individuals with PoMS were identified via a validated case definition. The index date ('MS onset') was the first demyelinating or MS specific claim recorded ≤18 years of age. We estimated the age-standardised annual incidence and prevalence of PoMS, and 95% CIs between 2003 and 2019. We used negative binomial regression models to assess the temporal changes in the annual crude incidence and prevalence of PoMS, and the ratios comparing sex groups. RESULTS: From 2003 to 2019, a total of 148 incident PoMS cases were identified in BC, and 672 in Ontario. The age-standardised annual incidence of PoMS was stable in both provinces, averaging 0.95 (95% CI 0.79 to 1.13) in BC and 0.98 (95%CI 0.84 to 1.12) in Ontario per 100 000 person-years. The incidence ratio by sex (female vs male) was also stable over the study period, averaging 1.5:1 (95% CI 1.06 to 2.08, BC) and 2.0:1 (95% CI 1.61 to 2.59, Ontario). The age-standardised prevalence per 100 000 people rose from 4.75 (2003) to 5.52 (2019) in BC and from 2.93 (2003) to 4.07 (2019) in Ontario, and the increase was statistically significant in Ontario (p=0.002). There were more female prevalent PoMS cases than males in both provinces. CONCLUSIONS: Canada has one of the highest rates of PoMS globally, and the prevalence, but not incidence, has increased over time. Allocation of resources to support the growing youth population with MS should be a priority.