Rates of the mumps across Ontario have been on the rise, and not just among those who refuse to be vaccinated. Even those who have gotten the shots are getting mumps, and doctors think they now understand why.

New numbers from Public Health Ontario show there were 261 cases of mumps reported last year – the highest case count in the province since 2008.

Mumps typically causes painful swelling of the salivary glands around the jaw and cheeks that can last for a week of more, along with fever, aches and pains. In rare cases, the illness can cause meningitis or encephalitis, both of which can lead to seizures, hearing loss, or death.

While the infection can strike anyone at an age, the median age for illness last year in Ontario was 28. Approximately, 24 per cent of those infected were not immunized, while more than a third didn’t know their immunization status at all. The rest had received only one dose of the vaccine or the recommended two doses.

Mumps infections have been on the rise for several years across all of North America. Dr. Robin Taylor, the associate medical officer of health at Ottawa Public Health, says the disease is spread through saliva.

“Our saliva can carry the virus, so if you're sharing water bottles, or other drinks of food with somebody, you can pass it on,” she said.

In 2014, there was an outbreak among NHL players, including Canadian star Sidney Crosby, and then another outbreak among NHL players last year.

A new analysis from research at Harvard University finds that all these recent outbreaks are likely due to the fact that the mumps vaccine does not offer lifelong protection.

The mumps vaccine was introduced in the mid-1960s, which led to a steep drop in infections. But by the mid-1990s, vaccine experts noticed the waning immunity problem and began recommending children get two doses of the combined measles-mumps-rubella (MMR) vaccine.

The first dose is typically given at 12 months of age, while the second is given either at 18 months or between the ages of 4 and 6 years old.

Now, it appears that many of those born before 1992 who received only one dose of the vaccine are getting infected as their single dose wears off.

The Harvard researchers found that protection from the vaccine persists for an average of about 27 years after the last dose.

They estimate that 25 per cent of people in the U.S. vaccinated against mumps may lose protection within 7.9 years; 50 per cent within 19 years, and 75 per cent within 38 years.

They say their findings suggest that a third dose of the vaccine might help sustain protection among adults. The study appears in Science Translational Medicine.

Canadians who were born between 1970 and 1992 likely received only one dose of the vaccine and should consider getting a second dose, say public health experts such as Dr. Taylor.

“It's the 20-, 30- and 40-year-olds, or people who didn't grow up in Ontario, who need to check,” she says.

Those who are not sure if they received one or two doses of the vaccine can check with their doctor or local public health unit. If there is still uncertainty, your doctor may recommend a booster shot.

With a report from CTV Ottawa’s Claudia Cautillo