Montreal's public health authority is trying to track down anyone who was in one of six places, most of them in Montreal's West Island last week, as they might have been exposed to the measles virus.

Dr. Mylène Drouin, the director of public health for the Montreal region, said the investigation is linked to two new cases of measles, linked to another case which the public health authority learned of on May 2.

Drouin estimates as many as 200 people may have been exposed to a person contaminated by measles over a period of three days — May 11, 12 and 14.

"​Every case of measles is an outbreak for us because we have to stop the chain of transmission," Drouin told reporters Thursday afternoon. "But we are more preoccupied by these two cases because we see that there's a transmission, and of course, we need to find those contacts really rapidly."

The places where people might have been exposed to the virus are:

Des Sources high school, 2900 Lake Street, on May 11 between 12:00 and 1:05 p.m.

Tim Hortons, 3760 Sources Boulevard, on May 11, between 12:25 and 13:35 p.m.

Royal Bank, 4400 Sources Boulevard, on May 11, between 12:15 and 13:25 p.m.

Manju Beauté, a beauty salon, 3637 Sources Boulevard, on May 11, between 2 p.m. and 5 p.m.

The Courtyard Marriott Hotel, 7000 Place Robert-Joncas in Saint-Laurent, on May 12 between 3 p.m. and 5:30 p.m.

Les Aventuriers daycare at 196 Bates Road, in Outremont, on May 14 between 4:30 and 5:35 p.m.

These latest cases bring to seven the number of measles cases reported in Montreal this year. Drouin said these two latest cases mark the first time in 2019 that the highly contagious virus has spread from one person to someone else.

Drouin said a girl contracted measles from a relative who caught the virus while travelling abroad. The girl was put in isolation, but she transmitted the virus to a health care worker who was treating her.

Drouin said that health care worker had been vaccinated for the measles, but the vaccination wasn't effective.

"We know that the vaccine is very effective," Drouin said. "The first dose is 85 per cent effective, and the second dose brings the efficacy to 95 per cent. But there are still some people who don't develop an immunity."

"In cases like this, it's the herd immunity that makes all the difference when it comes to protecting the population."

Those who may have contracted the virus at the Des Sources school on May 11 would have been children attending Iranian language classes, offered on Saturdays by the Iranian Cultural Society of the West Island.

About 60 children attend those classes, a society official said, and they and their parents have all been informed about the possible exposure.

How to know if you are protected

Public health is asking anyone who might have been exposed to the virus to check to see if they have been vaccinated and if that vaccination was sufficient.

You are considered protected from the measles if:

You had the measles, as confirmed by a doctor. (That includes anyone born before 1970, who is considered to have immunity due to exposure to the virus.)

You were born between 1970 and 1979, and you received one dose of the MMR (measles, mumps and rubella) vaccine.

You were born in 1980 or later, and you received two doses of the MMR vaccine.

Anyone who may have been exposed to the measles virus and is not protected is advised to call the INFO-SANTÉ line at 811.

Drouin said that the public health authority is in the process of setting up vaccination clinics. More information will be available Friday.

What are the symptoms?

Measles is highly contagious and is most often spread when people first get sick or before they know they have measles.

Symptoms include fever, cough, runny nose, stuffy nose, red and watery eyes and tiny white spots in the mouth.

Two to four days after the onset of symptoms, a rash including pimples and red spots appears — first on the face, then on the trunk, arms and legs, for three to seven days.

Measles can cause serious health complications such as pneumonia and encephalitis (inflammation of the brain), which can cause seizures, brain damage or death.

Complications are more common in people with weakened immune systems, pregnant women and children under one year of age.