Do I need a third dose of the measles vaccine? And other common questions answered

It's only April, and 2025 is already shaping up to be the worst year for measles since the disease was declared eliminated in 1998 in Canada.

Ontario continues to be the epicentre of the spread. Public Health Ontario is reporting 1,018 confirmed and probable cases of measles so far this year. The vast majority of those infected were not fully immunized. Six unvaccinated children and adolescents have been hospitalized with the virus.

In Alberta, too, the virus is spreading. The province is reporting 129 cases as of April 24, in every zone in the province. In that province, most of those sick are not fully vaccinated. Nine people have been hospitalized. 

Measles is one of the most contagious diseases known to man. Symptoms can include a fever, coughing and a runny nose, as well as a blotchy red rash. In serious cases, measles can cause complications like inflammation of the brain, pneumonia and death. 

As the virus spreads, here are some common questions about immunity to measles, answered. 

Can measles immunity wane?

Two doses of the measles, mumps and rubella (MMR) or measles, mumps, rubella and varicella vaccine (MMRV) offer almost 100 per cent effectiveness in the long term.  

Even so, immunologist Dawn Bowdish says she's been fielding questions about waning immunity from those worried they are not perfectly protected from the virus.

"Most of us who were born after 1970 have never encountered measles; we've only been vaccinated. We know that a small percentage of people in that scenario will have some decrease in immunity," she said. 

That could mean some breakthrough infections can happen among those who received two doses of a measles vaccine. 

It's a numbers game: the more measles is spreading, the more likely it is that a breakthrough infection happens, says Dr. Lynora Saxinger, an infectious disease specialist at the University of Alberta.

"People who have had two doses are substantially protected, but not bulletproof."

Even when those who are fully vaccinated get an infection, they're less likely to suffer the most serious consequences, says Dr. Dale Kalina, chief medical officer at Brant Community Healthcare System in Brantford, Ont., an area that has had measles cases since January.

"The reality is [those who've received two shots] aren't the people I would be worried about in an outbreak, from a spread perspective, nor from a complications perspective or a need for hospitalization," he said. 

How can I check whether I am protected against measles?

There is a blood test that looks for antibodies against the measles. Whether to get it or not is a conversation people need to have with their family doctors, says Bowdish.

Health-care workers or those who work with vulnerable people often get tested for antibodies, as do some pregnant women. Bowdish says she'd like to see that be the standard of care across the country, because measles used to be a major cause of lost pregnancies, stillbirths and birth defects in areas of the world with patchy vaccination rates. 

"As we move to an era where we have a lot more measles around, I personally am very supportive of pregnant women, or people who are planning on starting a family, getting this test."

WATCH | Getting your child vaccinated: 

How early can you get your child vaccinated against the measles virus?

24 days ago

Duration 3:49

Measles cases in B.C. remain low, with just five confirmed cases — all of them travel-related. But elsewhere, the numbers are exploding. Experts warn of further spread due to low vaccination rates. CBC's medical columnist, Dr. Melissa Lem, says she's been fielding questions from concerned parents. She says school-age kids are particularly vulnerable, and the virus can leave lasting damage to lungs and the immune system and even cause brain damage.

Those who are not sure if they've been fully vaccinated, though, are better off getting another dose, instead of getting the blood test, says Bowdish. 

"If you can't find your vaccination status, if you came from parts of the world that don't have the childhood measles [vaccinations], if you're worried that your parents might not have vaccinated you as a child — our recommendation is just get vaccinated," she said. 

Those who are born before 1970 are generally believed to be protected, says Bowdish, because they were exposed to measles as children. 

Do I need a third dose of the measles vaccine?

Canadian public health authorities don't recommend a third dose of the MMR vaccine for the general public, even though it's generally safe. That's because it wouldn't make sense to prioritize the limited time and public health resources we have to give a third dose right now, Bowdish says. 

There are some exceptions, like those who are on immunosuppressive drugs, or cancer treatments, Bowdish says. But those decisions should be made after a conversation between the patient and their doctor. 

Saxinger agrees.

"Making sure that ... people who are unvaccinated get vaccinated first is kind of the clear priority. There might be a time when we would start looking at the utility of additional doses for more people, but I don't know if it's quite now."