Montreal health authorities are trying to track up to 400 people they think may have been exposed earlier in the week to the measles virus.

Authorities have confirmed two secondary cases of the virus linked to a child who contracted the disease abroad, the city's public health director, Mylene Drouin, said Thursday.

Anywhere between 200 and 400 people could have been exposed between May 11-14, she told reporters.

Drouin published a list of locations the infected people visited along with the time they were there. She said anyone who believes they were in the same locations at the specified times should verify if they have been vaccinated and monitor themselves for symptoms of the illness.

Symptoms of measles include high fever, runny nose, coughing, conjunctivitis and general discomfort.

"We are trying to make sure that we find all the possible contacts and people at risk that may have been in contact with those secondary cases in Montreal," Dr. Mylene Drouin said, adding anyone who isn't protected should seek treatment.

The locations include a school, a restaurant, a bank, an esthetician, a hotel and a daycare centre.

Health Canada has reported 48 cases of measles in Canada between Jan. 1 and May 4, 2019. The cases were confirmed in Quebec, British Columbia, the Northwest Territories, Ontario, Alberta, and New Brunswick.

Measles is a highly contagious disease caused by a virus that spreads through the air and originates from the nose and throat of an infected person.

Since the beginning of the year, seven cases of measles have been reported in Montreal, but the first five involved exposure to the disease outside country.

Authorities said two people contracted the virus after they came into contact with an infected child on May 2.

One person was a member of the child's family. Authorities say that case is under control after the family member was isolated while they recovered from the illness.

The second person infected is a health care worked who treated the child. The employee had been vaccinated against the virus but it didn't provide the expected immunity.

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

The head of Quebec's College of Physicians, Dr. Yves Robert, wrote an open letter on the governing body's website this week promoting the effectiveness of immunization.

Robert wrote that immunization has made it possible to eradicate smallpox and control the spread of other diseases such as diphtheria, rubella and the measles.

"Vaccine efficacy and time unfortunately have an important side effect: forgetfulness," Robert wrote. "We no longer remember the risks of what we wanted to prevent and we only talk about the side effects of the preventive tool, real or hypothetical."

Quebec's last major measles outbreak dates back to 1989 when more than 10,000 cases were reported.

Drouin urged people to pay close attention to the dates, times and locations posted on the authority's website to determine if they were exposed.

A complete list of locations and times was posted Thursday to the city's public health department website.