A recent outbreak of mumps in Canada underscores the importance of getting the recommended two doses of measles-mumps-rubella (MMR) vaccine and not just one, according to a study.

The study on the 2009-2010 mumps outbreak, published in the Canadian Medical Association Journal, showed that almost three-quarters of people affected had received either no MMR shot or only one dose.

At particular risk were teenagers and young adults ages 15 to 24, who accounted for more than the 134 outbreak cases. In rare cases, mumps in adolescent and adult males may cause sterility.

Many people in that age group would not have gotten a second MMR dose, which was not recommended in Canada until 1996, and would have more chances to be exposed to mumps in either school or by living in a university dormitory.

"We need to ensure that older adolescents and young adults are up-to-date with their second dose, especially before they go to university," said Shelley Deeks of the Ontario Agency for Health Protection and Promotion in Toronto, who led the study.

After MMR became available in the 1970s, officials in the United States, Canada and elsewhere began recommending a single dose of the vaccine to protect children against mumps, measles and rubella.

But after some disease outbreaks in the 1980s, U.S. health officials in 1989 added a second MMR dose for children aged 4 to 6. Canada introduced a second dose in 1996.

Based on cases in the recent mumps outbreak, Deeks and her colleagues estimated the effectiveness of two-dose MMR at anywhere from 66 percent to 88 percent, depending on the age group. That was higher than the effectiveness of one dose, which ranged between 49 percent to 81 percent.

"Outbreaks occurring in Ontario and elsewhere serve as a warning against complacency over vaccination programs," Deeks wrote.

No one is sure how long mumps protection with the two-dose MMR lasts, and since recent outbreaks have included older teens and young adults who did have two doses of the vaccine, some researchers have suggested a third dose may be needed.

One issue with the vaccine is cost. While Canada has universal health care, in the United States the price of each shot for families without health insurance ranges from about $65 to upwards of $100.

Some U.S. doctors don't offer certain vaccines because of the cost, which many insurers don't sufficiently cover.

In addition, some parents have refused to get children vaccinated at all, in part due to now-discredited research that linked the MMR vaccine to autism.