The province is asking people who were at the "Acquire the Fire" gathering held at the Queensway Cathedral on Feb. 6 and 7 to check their immunization status as the number of confirmed cases in Ontario continues to rise.
One of five people in the Niagara Region confirmed to have contracted measles was at the event, said Ontario's acting chief medical officer, Dr. Robin Williams.
Health officials in Niagara said all five cases in the area — including three new ones announced Monday — are connected.
The person who went to the event "obviously didn't travel with what they knew to be measles disease, it was during the (infectious period)," Williams said. "And of course you get more infectious as you get closer to actually blossoming in the disease."
"There is some risk to those who were unimmunized at Acquire the Fire, and that's why we're reaching out today to make sure that people get on top of their immunization," she said.
Organizers with Acquire the Fire say roughly 1,000 people were present for the 27-hour event, described on the company's website as "the weekend that changes your teen's life forever."
Health officials say participants, as well as performers and volunteers, came from all over Ontario.
Individuals born after 1970 who were at the gathering are being asked to review their immunization status to ensure they are protected against measles.
The majority of teens in Ontario have received two doses of the measles vaccine, and for most, that is "highly effective" at preventing the illness, the province said.
Measles symptoms begin seven to 18 days after exposure, according to the Public Health Agency of Canada.
Those who took part in the event and develop symptoms of fever, cough, runny nose, inflammation of the eyes or rash over the next two weeks are being advised to phone their doctors.
The warning came as five new cases of measles were announced Monday in the province. In addition to the three new patients in the Niagara area, one new case was confirmed in Toronto and one in nearby Halton Region, for a total of 16 so far.
The Toronto case — a child whose vaccine history is unknown — is not connected to the youth gathering, Toronto Public Health said in an email.
The Halton case is a man in his 30s who recently travelled to Toronto, officials said, adding they could not confirm any link to other cases at this time. They said he did not spend any time in Halton Region during his infectious period.
Officials in Niagara Region, meanwhile, urged people who attended Our Lady of Mount Carmel elementary school on Feb. 9 and 10 or Saint Michael high school between Feb. 6 and 10 to review their immunization records. Both schools are in Niagara Falls.
Health officials in Quebec confirmed on Wednesday that 10 individuals in the Lanaudiere region northeast of Montreal had been infected with measles, with all cases linked to the outbreak at Disneyland in California.
On Tuesday, the Manitoba government reported the province's first case of measles of the year, a Winnipeg infant who had recently returned from India.