17 people now infected amid mumps outbreak, Toronto Public Health confirms

The condition is "very painful" and, in rare cases, can lead to sterility or decreased reproductive capacity, said Ian Culbert, executive director of the Canadian Public Health Association.

"It's always concerning to see outbreaks of vaccine-preventable diseases because we have the tools to prevent them," Culbert said.

He noted that when the vaccine for mumps was first introduced in 1970, the initial recommendation was for a single dose — but two doses has been the recommendation since 1996.

Anyone born before then may be "underimmunized" and at a higher risk for infection, Culbert said.