Ontario health officials have issued a warning after five new cases of measles emerged in three different regions.

One new case was confirmed in Toronto, another in nearby Halton Region and three new cases were lab-confirmed in the Niagara Region. So far, a total of 16 lab-confirmed cases have been reported in the province.

Attendees at an event at a Toronto church were exposed to the virus, Acting Chief Medical Officer of Health Dr. Robin Williams said in a statement issued Monday morning.

The statement warned that anyone who attended the Acquire the Fire events at Church on the Queensway, located at 1536 The Queensway, may be at risk.

The event was held at the church, formerly called the Queensway Cathedral, on Feb. 6 and Feb. 7. Williams said that a person from the Niagara Region who has the virus attended the event during the measles infectious period.

"That case was actually in their incubation period, so they were in their period when they could infect others," Williams told CTV Toronto.

Individuals who attended the events, and are born after 1970, are requested to review their immunization status to ensure they're protected against the measles, the statement said.

Anyone who has not been immunized is requested to "self-isolate" and contact public health officials, or call Telehealth Ontario at 1-866-797-0000 FREE. They're asked not to attend a doctor's office or health centre without calling first to warn of the risk.

"They should not attend any public gatherings and should not attend school, work, daycare or any post-secondary institutions until they are cleared of measles risk," the statement said.

Toronto Public Health said that the new Toronto case is not connected to the church event.

Niagara Region verifies 5 cases

Niagara Region Public Health confirmed Monday that, to date, there have been five-laboratory-confirmed cases of measles in the area.

The health authority confirmed the number in a statement posted to its website.

"Measles is now circulating in Niagara – all residents are urged to have their vaccination up-to-date," the statement said. Anyone who attended Niagara Falls' Our Lady of Mount Carmel elementary school on Feb. 9 and 10 or Saint Michael high school between Feb. 6 and 10 are being asked to review their immunization records.

Symptoms of measles include fever, cough, runny nose, inflammation of the eyes and rash. If individuals who attended the event develop one or more of the symptoms in the next two weeks, they're asked to contact their doctor and their local public health unit.

Niagara Region officials said that the three new lab-confirmed cases are linked to two earlier cases.

The latest cases of measles brings the provincial total to 16. None of the patients in Ontario had recently travelled outside the country, which means they contracted the virus in Canada.

Infectious disease expert Dr. Neil Rau told CTV News that despite the rise in measles cases in Ontario, we haven't seen a huge spike yet, meaning that most of the population is likely immunized.

"For every case that we describe we don't seem to have a huge number of additional cases," he said. "When you're dealing with an unimmunized population, one case will give rise to hundreds of cases."

With a report from CTV Toronto's Ashley Rowe

Public health advisory