Despite being on pace for the worst mumps outbreak in Toronto’s recent history, health officials say the public shouldn’t be overly alarmed with the community spread of the highly contagious disease.

As of Thursday afternoon, there were 31 confirmed cases of mumps in Toronto, nearly as many as the record-breaking 2009, when there were 33 cases over the entire year. Toronto’s outbreak accounts for a majority of the 41 confirmed cases in Ontario.

Toronto Public Health is investigating the mumps outbreak, which has largely affected 18- to 35-years-olds who frequented bars in parts of the city’s west end, specifically west of Yonge St., east of Lansdowne Ave. and south of Bloor St. W.

While this year’s outbreak may seem high, since the mumps vaccine was approved in Canada in 1969, the number of confirmed cases has decreased by more than 99 per cent, from an average of 34,000 in the 1950s to an average of 79 cases per year between 2000 and 2006.

Dr. Vinita Dubey, a spokesperson for the city’s health authority, said while the origin of the current outbreak is unknown, it’s likely that someone, or many people, travelled to a place where mumps was circulating and brought it back to Toronto.

Toronto Public Health says a broader community spread is now occurring outside the west end of downtown. Four of the 31 cases are related to staff or students at Toronto schools.

Sarah Wilson, a medical epidemiologist with Public Health Ontario, said few mumps cases are reported each year in the province — between 13 and approximately 40 cases confirmed annually in the last five years. But, every few years, there is a rise and resurgence in cases.

Why that happens could be for several different reasons, but Wilson said a major factor is that many people in their 20s and 30s have not received two doses of mumps-containing vaccines.

Two doses of a mumps-containing vaccine — either MMR or MMVR — offer 88 to 90 per cent effectiveness of preventing mumps infection. One dose is less effective, Wilson said, at about 78 per cent.

“In recent years, these cycles of increased mumps activity are occurring in a group that we know have increased susceptibility to mumps,” Wilson said.

In 1996, the province introduced a second dose of the MMR vaccine at Ontario schools. While many students received a second measles vaccine, many at that time — between kindergarten and high school — did not receive a second MMR vaccine.

Today, some people in their 20s and 30s may not have received a second dose.

“You have a group of people, around 25 to 37, who are under-vaccinated,” said Dr. David Fisman, a professor of epidemiology and medicine at the University of Toronto.

“Mumps has gone from being a disease of childhood to a disease of young adults.”

Another issue when it comes to controlling mumps is that classic symptoms — such as a swollen side of the face — do not always appear.

Jason Tetro, a visiting researcher at the University of Guelph and specialist in microbiology, said in some cases mumps symptoms can take up to two weeks to appear.

“When you hear about an outbreak, what often ends up happening is you’re already two weeks late,” he said. “When the outbreak happened in west downtown, that could take you to mid-January. We just can’t keep track of it in real time.”

The highly contagious virus spreads through coughing, sneezing and coming into contact with a person’s saliva.

When it comes to the 41 people in Ontario that have been diagnosed with the mumps virus, Wilson said the immunization records have been a “mixed picture” — some have no immunization record, while others have received one or two doses of a measles vaccine.

In Toronto, of the 31 cases, seven were fully immunized (have received two doses of mumps containing vaccine) and 24 received either one dose, no doses or unknown doses of the mumps containing vaccine.

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In the meantime, experts and public health officials are all urging parents and children born after 1970 to check their immunization records, and if they are unsure, to get a booster vaccine.

“The most important thing for people to keep in mind is that yes, we’re seeing more mumps activity, but there are things people can do to protect themselves,” Wilson said.

“Speak to your health-care provider, and make behavioural changes.”