Canada's loss of measles elimination after 27 years: a mathematical modelling analysis of the five major provincial outbreak waves in 2025-2026

Background . Canada's loss of measles elimination status in 2025 highlighted the vulnerability of populations with concentrated immunity gaps. Provincial incidence alone cannot readily distinguish relatively intense transmission within a small susceptible population from transmission involving a larger population. We quantified these differences across five provincial measles waves in 2025-2026. Methods . We fitted a common susceptible-exposed-infectious-recovered (SEIR) model to weekly reported measles incidence for Ontario in 2025 and Alberta and Manitoba in 2025 and 2026. Mean latent and infectious periods were fixed at 10 and 8 days. We estimated the effective reproduction number (R 0,eff ), initial effective susceptible population (S 0,eff ), and profiled the initial infected number to evaluate province and wave-specific primary case model estimates. We calculate the initial exponential growth rate, doubling time, and peak full width at half maximum (FWHM) to characterize early epidemic growth and the duration of elevated incidence around each wave's peak. Findings. In 2025, Ontario's effective reproduction number, R 0,eff , was 1.9, with a model-estimated effective susceptible population, S 0,eff , of 3,101 individuals, an initial doubling time of 2.4 weeks, and a peak FWHM of 12.3 weeks. Alberta had a similar R 0,eff of 1.9, with S 0,eff of 2,601 individuals, a doubling time of 2.4 weeks, and FWHM of 12.1 weeks. Manitoba had a lower R 0,eff of 1.5 and S 0,eff of 490 individuals, with a longer doubling time of 4.0 weeks and FWHM of 20.5 weeks. In 2026, Alberta's R 0,eff increased to 2.1 while S 0,eff decreased to 383 individuals; doubling time shortened to 2.1 weeks and FWHM to 9.7 weeks. Manitoba's R 0,eff increased to 1.8 and S 0,eff to 1,038 individuals, while doubling time shortened to 2.6 weeks and FWHM to 13.0 weeks. Interpretation. Alberta's smaller second wave combined the highest effective reproduction number with the shortest doubling time and peak duration, demonstrating that limited outbreak size can coexist with rapid transmission. Joint estimates of transmission intensity, epidemic pace, peak duration, and effective susceptible population provide complementary measures for comparing outbreaks and informing public-health planning. Together, these estimates provide a quantitative basis for distinguishing transmission intensity from the scale of susceptible involvement, supporting comparisons across outbreaks and informing the timing and scale of public-health responses for future outbreaks.