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. 

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. 

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.