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"Normally, in an outbreak (it) would go in a very escalating, rapid fashion, but we're seeing stable numbers week after week, which gives us hope," he said.

"This virus typically spreads in late winter and through spring, and we're finding that it's not accelerating. So that's probably thanks to the great work in communities to try to limit spread, as well as our messaging with local public health agencies to affected communities and health system preparedness." 

Public Health Ontario says there have been 572 cases since the outbreak began in October — 453 of them confirmed and 119 probable. Of the 42 people requiring hospitalization, two have required intensive care, and 36 have been children — most of them unvaccinated.

Moore wrote in a memo to local medical officers of health earlier this month that the measles outbreak is "disproportionately" affecting some Mennonite, Amish and other Anabaptist communities due to under-immunization and exposure. The origin of the outbreak was a large gathering with guests from Mennonite communities in New Brunswick last fall, he wrote.

He said Friday that the "vast majority" of Ontario's cases are among people in those communities, and local public health units are focusing on outreach to try to contain the spread. Unvaccinated infants, kids and teenagers in the Southwestern and Grand Erie public health units are most affected. 

"(The health units) have always had a strong relationship with these communities, and have continued the communication," Moore said, including communications in Low German, spoken by some Mennonites. 

"They have advertised on some of the Low German radios. They've translated their documents into Low German to ensure that they can be understood at community level...We call it the ground game. The ground game outbreak is where all the work has to happen and all the communication and collaboration."

The Ontario Liberals have urged Moore and Premier Doug Ford to more publicly deliver public health messaging on measles.

"The premier and his chief medical officer of health need to tell the people of Ontario their plan to fight measles," public health critic Adil Shamji wrote in a statement.

"The government of the day should not have to be told to take action. It should be leaping into action. Anything less will put thousands of lives at risk."

Moore said he is not planning a press conference. Local medical officers of health have been the public lead on the issue, and some medical experts have been doing television interviews about measles, he said. 

"We know our partners at Public Health Ontario are experts that are helping us with this outbreak, (and) have had good communication," he said.

"We monitor the effectiveness of our communication, and we're happy with how this is being covered, how the press has really raised the awareness of this, both at a national level and within Ontario."

There have been measles cases outside the most-affected health units, but many are linked to global travel and have not spread locally, Moore said.

Measles is one of the most contagious diseases in the world. The World Health Organization says the virus can remain active in the air or on infected surfaces for up to two hours.

It usually begins with a fever, cough, runny nose and red watery eyes, followed by a red blotchy rash that starts on the face and spreads to the body and limbs. The virus can lead to pneumonia, inflammation of the brain and death.

The number of cases reported in Ontario over the last week is more than the number of cases recorded over the course of a decade between 2013 and 2023.

- With files from Hannah Alberga

This report by ºÃÉ«tvwas first published March 28, 2025.