With measles cases on the rise in Canada, some adults may be questioning whether their immunity, either through natural infection or vaccine, is still strong enough.
While many Canadians receive two doses of the measles vaccine in childhood, those born after 1970 may have only received one — and their protection could have weakened over time.
Routine measles vaccinations didn’t begin in Canada until 1970, and even then, a second dose wasn’t added to the schedule until 1996, leaving some adults with incomplete protection.
“People born in, for example, 1971 aren’t that young anymore, and they might have received two doses of a measles vaccine or one dose of a vaccine, but they may not have the same degree of immunity,” said Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital.
“There probably are a lot of people who think they’re vaccinated who have only had one dose, and the recommendation is if you’re traveling to places where there’s a lot of measles circulating and you only had one dose, you should get a second dose,” Bogoch added.
Measles is one of the most contagious infectious diseases — more contagious than diseases like COVID-19, influenza and chickenpox. This high level of contagiousness is one reason why measles outbreaks can spread rapidly, particularly in areas with low vaccination rates.
So far this year, Canada has reported 95 cases of measles (as of Feb. 21), with British Columbia, Manitoba, Ontario and Quebec bearing the brunt of the outbreak, the latest data from the Public Health Agency of Canada (PHAC) shows.
The measles vaccine is available in Canada as measles-mumps-rubella (MMR) or measles-mumps-rubella-varicella (MMRV) vaccine. It’s estimated to be 85 to 95 per cent effective after a single dose given at 12 or 15 months of age. With a second dose, efficacy rises to nearly 100 per cent, according to PHAC.
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With March break around the corner and measles outbreaks surging in many parts of the world — including a recent tragic case in Texas where a child died — experts are urging Canadians to ensure their measles vaccinations are up to date.
“So I think in order to prevent measles, vaccine is the most effective means and right. The routine immunization recommendation is two doses for people who are born after 1970. So you should really take every opportunity you get to check to see.”
Who should get the measles vaccine?
Canada introduced the measles vaccine in 1963, but routine childhood immunization didn’t begin until 1970 with a single dose.
Those born before 1970 were generally assumed to have natural immunity from prior exposure.
However, PHAC says certain groups, including healthcare workers, travellers and military personnel, should receive a measles vaccination regardless of their birth year.
In 1996, Canada implemented the two-dose MMR vaccine schedule, offering a second dose through catch-up campaigns for school-aged children. Since then, measles cases have dropped by more than 99 per cent, though outbreaks can still occur when unvaccinated people travel to countries where measles is circulating.
While vaccination rates in Canada are high, they’re not sufficient to maintain community immunity in some areas (a herd immunity rate of 95 per cent is needed), which can lead to outbreaks from imported cases. For optimal protection, PHAC recommends:
- Routine childhood immunization: two doses of any measles-containing vaccine, with the first dose at 12 to 15 months and the second at 18 months or later.
- Unvaccinated children and adolescents: two doses of measles-containing vaccine.
- Susceptible adults born after 1970: one dose of measles vaccines (two doses for those at higher risk, such as travellers and health-care workers).
- Susceptible adults born before 1970: two doses of MMR vaccine for health-care workers, military personnel and travellers to high-risk regions.
Sabina Vohra-Miller, founder of the educational platform Unambiguous Science, recommends that if you’re travelling — especially during March break — you should ensure you’re up-to-date with your measles vaccine and consult your health-care provider.
“Those who are travelling, for instance, to an area that has high exposure risk to measles can also consider getting a second dose of vaccine if they feel that they’re unvaccinated, or they’re not sure whether they’ve gotten two doses or not,” she said.
She said if you are unsure whether you are adequately protected, you can talk with your family doctor and you can get that extra dose of MMR vaccine.
“It does not hurt to get an MMR booster because the vaccine is incredibly safe and incredibly effective,” she said.
What about a booster?
As measles cases rise in Canada, experts say it’s still too early to determine whether a third booster shot will be needed for those who may require a refresher.
For now, a full series means you have had two shots.
“I don’t think the entire Canadian population who’s already received two doses requires a booster,” Vohra-Miller said.
“What is important to remember is that measles tends to be a much milder disease in those who are already vaccinated, and they’re also far less likely to transmit the virus if they’re already vaccinated,” she added.
