Measles cases have rapidly increased in Canada since the start of the year, with the majority reported in Ontario.

According to Public Health Ontario, 277 confirmed cases and 69 probable cases of measles have been logged in the province this year as of Wednesday.

The organization said all but 11 cases were associated with an ongoing multi-jurisdictional outbreak, five cases had a history of travel and were acquired outside of Canada and one was epidemiologically linked to a visitor to Ontario.

Of these, three cases required hospitalization, all among unimmunized children.

“In the past, cases in Ontario have been primarily associated with travel, meaning they acquired measles outside of Canada. However, there is currently a measles outbreak in Ontario, primarily southwestern Ontario, and the majority of measles cases in 2025 have been acquired in Ontario,” Public Health Ontario warned in a statement.

Measles cases have spiked dramatically, increasing by 195 since the last report at the end of February, with seven new public health units reporting cases in their regions.

This is a massive rise compared with the 146 cases reported in all of Canada in 2024, and much more than the 12 cases reported in Canada in 2023.

1:48 Health officials in the Maritimes urging protection against measles as cases climb

In February, Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital, said we are not headed in the right direction, with roughly the same number of measles cases last month as in all of 2024.

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.

Public Health Ontario warns that the only way to protect oneself and prevent the spread of the virus is to stay up to date with your measles vaccinations.

The organization attributes the sharp increase in the number of outbreak cases and the geographic spread in recent weeks to ongoing exposures and transmission among unimmunized individuals.

Public health reports that individuals born in or after 1970 made up 99.2 per cent of outbreak cases, 72.6 per cent of cases were in children and adolescents, and 26.3 per cent were in adults.

1:46 Health Matters: Child tests positive for measles in Hamilton, Ont.

Of the cases involving children and adolescents, 90.7 per cent were unimmunized, while 54.1 per cent of cases were unimmunized adults.

The report shows that seven of the infected individuals were pregnant, of which five were unimmunized and two had previously received two doses of measles vaccine, and one case was in a newborn.

“It’s so transmissible; if someone was in a room that had the measles and then they left the room, and an hour or two later, another person entered that room who is not immune to measles, there’s a high probability that they would get infected with measles,” Bogoch warned.

“It has a knack for finding unvaccinated or under-vaccinated people and under-vaccinated communities.”

— with files from Global News’ Katie Dangerfield