Measles immunity 90% in BC’s Lower Mainland

(Press-News.org) In British Columbia’s Lower Mainland, 90% of people have detectable antibodies against measles, indicating high vaccine coverage and population protection, according to a new study published in CMAJ (Canadian Medical Association Journal) https://www.cmaj.ca/lookup/doi/10.1503/cmaj.250788.

Measles has been on the rise in North America, with more than 4000 cases reported in Canada as of August 2025, a fivefold increase over any full-year tally since Canada achieved measles elimination status in 1998. Measles is highly infectious, and herd immunity should be around 92% to 94% to prevent widespread circulation. Individuals born before 1970, during the time of endemic measles circulation, are considered immune through infection, while those born after the decline in measles circulation rely on vaccine-induced antibodies. Two doses of measles vaccine are recommended, with the first dose routinely given at 12 months and the second dose given at 4 to 6 years of age in BC.

In recent years, public health officials have expressed concern that the increase in vaccine hesitancy may be negatively affecting vaccine coverage. About 90% of the measles cases in Canada have involved unvaccinated people.

To gauge the risk of sustained measles transmission in the community at large, researchers analyzed blood samples collected in August 2024 from more than 1000 people within the most populated Lower Mainland region of BC. They found measles antibodies in about 90% of people overall, including 92% to 94% of 2- to 3-year-olds and 97% of 4- to 6-year-olds, reflecting high first- and second-dose measles vaccine coverage in children. Measles antibody levels were lower in older children, including those who had been due for their second dose of vaccine during the COVID-19 pandemic.

Virtually all adults aged 55 years or older but fewer than 75% of adults aged 20 to 39 years had detectable measles antibodies. The authors suggest that this may be due to a combination of factors, including a decline in vaccine-induced antibody with age that may be exacerbated in people vaccinated as infants who were born to previously infected mothers. Vaccinated people in whom antibodies cannot be detected likely still have some protection due to other components of the immune response.

“The observed epidemiology in BC and elsewhere in Canada indicates that current levels of population immunity are sufficient to suppress sustained or generalized measles transmission,” writes Dr. Danuta Skowronski, BC Centre for Disease Control and the University of British Columbia, Vancouver, BC, with coauthors. “This is supported by measles outbreaks that have been sporadic, self-limited, or centred on linked and largely nonvaccinated individuals or communities. … Ensuring high 2-dose coverage in the general population remains critical to elimination.”

The findings show that the vast majority of parents know the importance of vaccinating their children, with serosurveys helping to identify areas for improvement.

“Overall, our serosurvey in August 2024 identified a higher proportion of children with measles antibody, reflecting vaccine uptake, than vaccine coverage estimates would suggest. Our findings highlight missed documentation of administered measles vaccine doses and reinforce the importance of efficient and accurate systems to capture and confirm 2-dose coverage.”

“Periodic population-based serosurveys, interpreted within a public health context, can clarify estimates of vaccination coverage, inform vaccination catch-up and other program adjustments, and support measles elimination efforts overall,” concludes the investigative team.

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