Measles seroprevalence among individuals serologically tested in Ontario, Canada

Background — Canada eliminated measles in 1998, but the risk of outbreaks remains due to sub-optimal vaccine coverage and imported cases. To address limitations of vaccine coverage data, we measured measles seroprevalence among a convenience sample of individuals serologically tested in Ontario, Canada.

Methods — We linked measles IgG serology data from 349,451 individuals tested during 2014–2016 to health administrative databases and used a cross-sectional design to measure measles IgG seroprevalence. We estimated seroprevalence overall and by sociodemographic factors.

Findings — Using the manufacturer-recommended threshold of ≥275 mIU/mL, measles seroprevalence was 83.5 % (95% CI, 83.4–83.6 %), below the 90–95 % herd immunity threshold. However, using the often cited >120 mIU/mL threshold, seroprevalence was 97.1 % (95% CI, 97.0–97.1 %). Using a threshold of ≥275 mIU/mL, seroprevalence varied by age, with the lowest observed among adolescents aged 12–19 years (75.3 %; 95% CI, 74.8–75.7 %) and young adults aged 20–29 years (75.8 %; 95% CI, 75.5–76.1 %), and highest among the oldest cohorts aged ≥50 years (96.9 %; 95% CI, 96.8–97.0 %). When using a lower threshold of protection (>120 mIU/mL), large differences in seroprevalence by age were not observed. Among individuals born in or after 1985, immigrants had lower seroprevalence (≥275 mIU/mL) (71.0 %; 95% CI, 70.6–71.5 %) compared to those born in Canada (77.8 %; 95% CI, 77.6–78.1 %).

Interpretation — The uncertainty of the exact correlate of protection for measles complicates the interpretation of seroprevalence data when assessing whether this convenience sample of individuals serologically tested for measles is adequately protected to prevent future outbreaks. Measles seroprevalence varied significantly by age and immigrant status, with highest seronegativity (<275 mIU/mL) observed among adolescents, young adults, and young immigrants. It is possible that sustaining measles elimination in Ontario will be challenging in the face of ongoing measles importations from abroad, and immunity gaps in some population groups.