Seventeen cases of measles have been confirmed in Quebec, Ontario, Saskatchewan and British Columbia — more than half of those in the Montreal area, while one Ontario case has been linked to a high school.

Quebec public health director Dr. Luc Boileau confirmed 10 cases on Monday, almost all of them involving children and making Montreal the country’s epicentre. He said only three of the cases were linked to travel outside of the country, indicating community spread in and around the city.

Public Health Ontario confirmed five cases of measles and said all but one involved travel.

The latest case, announced last week, was in a man in his 30s in the York Region north of Toronto and was “likely” related to community transmission, the region’s medical health officer said.

The man had “close contacts” among students and teachers at a high school so public health officials in that region are ensuring everyone there is vaccinated, Dr. Barry Pakes said.

More than 1,500 students and 150 staff received notification from public health officials on Feb. 29 that they were exposed to a positive case of measles, he said in an email.

Pakes said they were invited to an immunization clinic on Sunday and had “excellent turnout.” The school has a 95 per cent measles vaccination rate “and climbing,” he added.

Students without proof of two doses of the measles, mumps and rubella vaccine will not be allowed to return to school until March 15, Pakes said.

He said the man in his 30s was fully vaccinated.

Dawn Bowdish, an immunology professor at Hamilton’s McMaster University, said that it’s rare for someone to be infected after getting vaccinated.

“When I hear about a case like this, I’m thankful this person was vaccinated because it means they probably had a less dangerous course (of illness) than they might have had,” she said, noting that measles is not only the most contagious known virus, but also has the “highest death rate of all the vaccine-preventable infections.”

“The patient would also be less likely to spread measles to others than if he had been unvaccinated,” she said.

B.C.’s provincial health officer Dr. Bonnie Henry said one case of the virus, related to international travel, was recorded in the Vancouver Coastal health region on the weekend in a child younger than 10. A case in Saskatoon, confirmed in January, was also linked to travel abroad.

British Columbia and Ontario last reported cases of the virus in 2019 when the western province mandated parents and guardians to provide public health units with immunization records for students enrolled in the public school system.

Infectious disease specialists say a small number of measles cases can escalate to widespread infections as it has elsewhere in the world, including Europe, where thousands of cases have been confirmed.

Henry urged parents to ensure children are vaccinated against measles, especially if they plan to travel during spring break, which starts next week in British Columbia. Her federal counterpart, Dr. Theresa Tam, issued the same message in a statement on Feb. 23.

B.C. Premier David Eby said confirmation of the single case in that province is “terrifying” for parents of infants. He issued a rebuke to people “trafficking in misinformation about vaccines.”

“Without those vaccines you’re putting infant children in our province at risk of very, very serious illness,” he said.

Symptoms of measles are similar to those of a cold or flu before a rash appears. The virus can lead to severe illness in children including pneumonia and swelling of the brain.

Health Canada said that by March 2021, national vaccination coverage rates were similar to those in 2019, with 92 per cent of two-year-olds vaccinated against measles.

Canada has set a target of 95 per cent cent coverage by 2025 for various childhood vaccines, including two doses of a measles-containing vaccine by about age seven.

— With files from Dirk Meissner in Victoria and Jacob Serebrin in Montreal.