The effect of age at first dose of a measles-containing vaccine on measles humoral immunity: a retrospective cohort study, Canada, 2014 to 2016

BACKGROUNDCanada eliminated measles in 1998; however, large outbreaks have been recently reported, and elimination status was lost in 2025.AIMWe aimed to estimate the effect of age at first dose of measles-containing vaccine (MCV1) on measles IgG concentration and seronegativity among two-dose recipients nearly two decades after vaccination.METHODSWe conducted a retrospective cohort study of 10,766 twice-vaccinated individuals tested for measles IgG from 2014 to 2016 in Ontario, Canada. We predicted measles IgG concentration and seronegativity using thresholds of < 275 mIU/mL, < 200 mIU/mL and ≤ 120 mIU/mL.RESULTSA 1-month increase in age at MCV1 (between 12 and 15 months) was associated with 13.7% (95% confidence interval (CI): 7.1-20.8) higher predicted mean measles IgG concentration for females born after elimination and 9.2% (95% CI: 6.8-11.6) for females born before elimination. Depending on the seronegativity threshold and the measles elimination status at birth, a 1-month increase in age at MCV1 (between 12 and 15 months) was associated with lower odds ratio of seronegativity ranging from 0.74 (95% CI: 0.63-0.85) to 0.87 (95% CI: 0.57-1.27). Using the lowest threshold (≤ 120 mIU/mL) and the longest follow-up period (24 years post-MCV1), we predicted that 4.1% (95% CI: 2.7-6.1) of females vaccinated with MCV1 at 15 months of age would be seronegative, compared with 9.6% (95% CI: 7.4-12.3) of those vaccinated at 12 months.DISCUSSIONDelaying MCV1 to older ages (13-15 months) may result in an absolute decrease as large as 6% in the percentage of seronegative individuals, potentially increasing the likelihood of maintaining herd immunity.