The province’s chief medical officer of health is urging Ontarians to make sure they are immunized against the measles, amid the most severe outbreak in the province in more than a decade.

In a statement Friday, Dr. Kieran Moore said there have been 350 cases of the virus in Ontario since late October — with 173 of those infections reported within the last several weeks — and 31 related hospitalizations.

“Over 96 per cent of cases in Ontario are among individuals who are unimmunized, or have unknown immunization status, and were exposed in their community or while travelling,” Moore said. He added that a majority of infections are concentrated in the southwestern part of the province among unvaccinated people.

“This is the most measles cases Ontario has seen in over a decade. It is critical we work together to mitigate further spread of this vaccine-preventable disease,” he said.

The statement noted the measles, mumps and rubella (MMR) vaccine has been used for more than 50 years and is “proven to be one of the safest and most effective vaccines available.” Children fully immunized with two doses of the vaccine are nearly 100 per cent protected, while a single dose provides roughly 95 per cent protection, Moore said.

The MMR vaccine is typically given between 12 to 15 months of age, though it can be given as early as six months in cases where there is higher risk of exposure. The booster — which also has protection against chickenpox — is given at four to six years old.

In the last school year, only 70 per cent of seven-year-olds were fully vaccinated against measles, Public Health Ontario data shows. The agency says this represents a “large decline” from rates before the COVID-19 pandemic, as seen with many of Ontario’s routine publicly funded immunization programs.

The National Advisory Committee on Immunization says 95 per cent is needed to ensure Canada stays free of endemic measles.

Almost all of the new cases in Ontario are connected to an interprovincial outbreak first reported in New Brunswick, which has also spread to Manitoba.

Most of the sick people are in the Southwestern Public Health region, where almost half of the cases are reported, and neighbouring Grand Erie, which has close to 27 per cent. But the highly contagious virus has also travelled hundreds of kilometres away from that area, spanning west to Windsor and east to Niagara.

“Measles spreads easily among those who aren’t vaccinated and can lead to serious health issues including pneumonia, respiratory failure, swelling of the brain, and in rare cases, death,” Moore said Friday.