The latest person to be infected with measles in Toronto was vaccinated against the disease, public health officials confirmed Thursday afternoon.

The individual — an adult — joins four other unrelated cases in Toronto. One woman in her 20s in the Niagara Region also contracted measles this week.

The latest case is the first to involve someone who was up to date with the MMR (measles, mumps, rubella) vaccination.

“No vaccine is 100 per cent effective,” said Dr. Shelley Deeks, medical director of immunization and vaccine-preventable diseases with Public Health Ontario. “There is always going to be a small proportion of people who don’t respond to vaccine.”

The MMR (measles, mumps and rubella) vaccine is considered to be over 95 per cent effective. This means some 5 per cent of people will still be vulnerable after receiving the recommended two doses of the vaccine.

Deeks explained that some people don’t mount an immune response to the vaccine and for others, the effectiveness wanes over time.

On Monday, Toronto Public Health announced a measles outbreak had hit the city , with four lab-confirmed cases of the highly contagious, potentially deadly disease.

The other cases include two children under the age of 2 and two adults. Three of the individuals were not vaccinated and one only received one dose of the intended two-dose vaccine.

Toronto Public Health has not yet found a link between the cases. No patients are from the same family.

“This indicates measles is currently circulating in Toronto,” Lenore Bromley, media relations and issues management manager, wrote in an email to the Star, adding that none of the infected people had travelled outside the country.

The Niagara Region woman, who travelled to Toronto in January, was also unvaccinated.

Bromley said the new case highlights the importance of “herd immunity” — meaning a high rate of vaccinations in a community will lower the chances of measles spreading.

Public health experts peg the effective herd-immunity vaccination rate at 95 per cent. In Ontario, vaccination rates have hovered around 95 per cent for 17-year-olds for the past five years.

MMR Vaccinations are not recommended for children under the age of 12 months or for people with compromised immune systems, such as people undergoing chemotherapy.

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Symptoms of the disease include a high fever, cold-like symptoms and a red rash that starts on the face and neck and spreads to the rest of the body. Complications can occur in 10 per cent of cases, including people pneumonia and ear infections. In rare cases, encephalitis, or swelling of the brain, can lead to death.

With a file from The Canadian Press

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