Public health officials are urging Torontonians to check their vaccination records after a measles outbreak with no known link between victims hit the city Monday.

Lab tests confirmed four people, including two children under the age of 2 and two adults, are infected with the potentially deadly disease, Toronto Public Health announced. One was hospitalized, but officials would not release other health or personal details, citing privacy reasons.

“The message is that measles is circulating in Toronto,” said Dr. Lisa Berger, associate medical officer of health with Toronto Public Health. “There has been spread somewhere.”

Berger said that in three of the four cases, the individuals had not been vaccinated against the disease. In the fourth case, the person received only one dose of the double-dose vaccination, she said.

None are from the same family and there is no source case, said Berger.

“It’s always concerning to me when we get cases of a disease that has been eliminated in Ontario,” said Dr. Shelley Deeks, medical director of Immunization and Vaccine Preventable Diseases with Public Health Ontario.

The last endemic case of measles in Canada was in 1997, with recent outbreaks the result of imported viruses from endemic regions, according to the Public Health Agency of Canada.

In Ontario, the number of reported cases per year has fluctuated from three to 22 over the past five years, said Deeks.

“Will I be surprised if we see subsequent cases from this? Absolutely not,” said Deeks.

The Toronto outbreak is the first in Ontario this year.

It comes just days after a measles outbreak that originated at Disneyland in December spread to 100 people.

Berger said none of the victims had travelled outside Toronto in the past 21 days, nor had they been in contact with anyone who had travelled.

Canadian children typically receive two doses of the combined measles vaccine — MMR (measles, mumps, rubella) and MMRV (measles, mumps, rubella and chickenpox) — which is more than 90 percent effective.

Deeks said that in Ontario, vaccination rates have hovered around 95 per cent for 17-year-olds for the past five years. However, Ontario has seen a “slight increase” of “conscientious objectors” to vaccines in recent years.

Jane Heffernan, associate professor in the Department of Mathematics and Statistics at York University and director of the Centre for Disease Modelling, said people may oppose vaccinating children because the effects of diseases are less visible today than they once were.

“We don’t see very many cases now, and that may, in some people’s minds, sway them to be more hesitant towards having their children vaccinated … because it’s hard to assess the risk of infection when you don’t see it,” she said.

Controversial anti-vaccine movements have gained attention in recent years, notably after actress Jenny McCarthy linked vaccinations with her son’s autism, which is not supported by scientific evidence.

On Monday, Health Minister Eric Hoskins said parents who don’t get their children vaccinated for measles risk boosting the spread of the "dangerous" disease.

"Parents really need to look at the evidence ... we need to continue to educate the population," he told reporters. "There are gains that can be made."

Ontario children must be vaccinated against measles to attend school — however, parents may submit a statement of conscience or religious belief to exempt children from vaccine requirements.

Toronto Public Health is urging Torontonians to check their medical records to ensure they are fully vaccinated. People who show symptoms should call their health-care provider and notify medical offices before showing up.

Measles is a highly contagious, potentially fatal disease. Symptoms include a high fever, cold-like symptoms, sore eyes or sensitivity to light and a red rash that starts on the face and neck and spreads to the rest of the body. Serious cases can result in pneumonia, blindness, and encephalitis — swelling of the brain — which can lead to death.

Most people recover within 10 days.

The TPH investigation into the outbreak is ongoing, said Berger.

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With files from Rob Ferguson

The 411 on measles

What is measles?

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A highly contagious respiratory infection caused by a virus. It is one of the most contagious among vaccine-preventable diseases.

How is measles spread?

Through coughing and sneezing, when an infected person releases droplets containing the virus in air.

When should people get vaccinated?

Children generally receive the first dose at 12 months and the second when they are 4 to 6 years old.

Who is at greatest risk?

Infants under 1 year old, pregnant women and people with weakened immune systems can become very ill. People born before 1970 may be protected, as they are likely to have developed immunity.

Can people who are fully vaccinated contract the virus?

There is a small chance; but the vaccine is over 90 percent effective.

If you have it once can you get it again?

No. You can only contract it once.

When is someone contagious?

Carriers are contagious from four days before to four days after a rash appears. A rash usually appears 10 to 14 days after exposure.

How can you avoid contracting the virus?

Ger vaccinated. Wash your hands with soap and water frequently. Cover your nose and mouth with your arm when sneezing and coughing.

What are the vaccination rates in Ontario?

According to Public Health Ontario, vaccination rates for 17-year-olds in Ontario have fluctuated between 94.4 percent and 95.4 percent since 2010.

Sources: Toronto Public Health, Public Health Agency of Canada, Public Health Ontario

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