A recent outbreak of mumps in Canada underscores the importance of getting the recommended two doses of measles-mumps-rubella (MMR) vaccine, researchers reported Monday.
A study investigating the 2009-2010 mumps outbreak 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 half of the 134 outbreak cases.
Many people in that age group would not have gotten a second MMR dose, which was not recommended in Canada until 1996. And compared with older adults, they would have more chances to be exposed to mumps -- from living in a college dorm, for example.
The findings underline the importance of having children get the recommended two doses of MMR, said lead researcher Dr. Shelley L. Deeks of the Ontario Agency for Health Protection and Promotion in Toronto.
In addition, she told Reuters Health, "We need to ensure that older adolescents and young adults are up-to-date with their second dose, especially before they go to university."
Deeks and her colleagues report their findings in the Canadian Medical Association Journal.
Mumps is a viral infection that causes fever, headache, muscle aches and swollen glands around the jaw. Most people recover in a few weeks, but a small number of patients can have serious complications, such as an inflammation of the brain that can be life-threatening, or hearing damage (usually temporary), or painful inflammation of the testicles or ovaries.
After MMR became available in the 1970s, officials in the U.S., Canada and elsewhere began recommending a single dose of the vaccine to protect children against mumps, measles and rubella (German measles).
But after some disease outbreaks in the 1980s, U.S. health officials in 1989 added a second MMR dose for children ages 4 to 6.
Canada introduced a second dose seven years later, in 1996.
In the past few years, there have been mumps outbreaks in Canada, the U.S. and other countries - and some of those outbreaks have included people who've had two doses of MMR.
"No vaccine is 100-percent effective," Deeks said. And researchers suspect that over a period of many years, the vaccine's protective effect might start to wear off, which could leave some people susceptible.
Based on cases in this 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 and 81 percent.
Deeks called the findings "reassuring," because the two-dose MMR was more effective -- and because none of the people who got sick during the outbreak were young children.
No one is sure how long mumps protection with the two-dose MMR lasts. And because recent outbreaks have included older teens and young adults who did have two doses of the vaccine, some experts have suggested that a third dose may be needed.
That's still an open question, Deeks said.
For now, she added, the important thing is to encourage older teenagers and young adults who have not had their second MMR shot to get it.
Even if a vaccine is not 100-percent effective, vaccinating enough people can still prevent large disease outbreaks. It's estimated that nine out of every 10 people must be vaccinated against mumps in order to achieve such "herd immunity."
Deeks said it's important that health officials and the public not become "complacent" about mumps. "All children absolutely should receive two doses of MMR, and keep up with all their vaccines," she said.
One problem the research team didn't address in their paper is the cost of the MMR vaccine - and it's not cheap. In Canada, which has universal health care, parents don't pay out-of-pocket for MMR shots for their children. In the U.S., however, the price of each shot for families without health insurance would range from about $65 to upwards of $100, depending in part on whether they took their kids to a walk-in clinic or a doctor's office.
Some primary care doctors in the U.S. don't offer certain vaccines because they're so expensive, and many insurers don't sufficiently cover the cost.
Another barrier to widespread two-dose vaccination: some parents don't let their children receive the MMR shots out of fear that the vaccine is linked to autism. However, numerous international studies have found no evidence that the vaccine contributes to autism.
One of the researchers who worked with Deeks on the study has received research funds from GlaxoSmithKline, one of the companies that makes the MMR vaccine.