A new U.S. study suggests the timing for the delivery of mumps vaccine or the number of shots given might need adjustment to avert or control future outbreaks of the disease.

The study, a report on a large mumps outbreak in the U.S. Midwest in 2006, revealed that 63 per cent of the people who came down with mumps had received the recommended two doses of mumps vaccine in childhood. That suggests waning immunity played a role in the outbreak.

Trying to shore up immunity against mumps into adulthood by delaying delivery of the second dose of mumps vaccine is a possibility, but not something immunization advisory bodies would consider without more research and evidence of need, the senior author of the study said in an interview.

That's because mumps vaccine comes mixed in a vaccine cocktail that also protects against measles and rubella, two diseases public health authorities view with more concern.

"I think suggestions for changing the schedule are something we've thought about, but it's not something we would do lightly at all," said Dr. Jane Seward, a vaccine expert in the division of viral diseases at the U.S. Centers for Disease Control in Atlanta.

"We wouldn't want to tamper with what's been a very, very successful program for these three diseases — with measles being the most concerning of the three — without considering the effect it would have on the other two antigens," she said, referring to the measles and rubella protection offered by the shots.

Outbreaks affect mostly young adults

The MMR vaccine, as it is called, is given in two doses in childhood. In both Canada and the U.S., authorities recommend the first dose around the first birthday, with the second dose given sometime before school entry. In some provinces, the second dose is given at 18 months.

Seward and colleagues from the CDC reported on the 2006 mumps outbreak that began and centred around Iowa. A total of 6,584 cases were registered, most in the first half of the year and clustering in eight adjoining states.

University campuses, where there is lots of close contact and plenty of opportunities for sharing of saliva, were hotbeds of mumps transmission. The highest proportion of cases were in people aged 18 to 24.

A year later a similar university-based outbreak erupted in Nova Scotia and New Brunswick. Cases eventually were logged in all 10 provinces and at least two — Alberta and British Columbia — are still in the midst of outbreaks.

But unlike the U.S. situation, in Canada the majority of cases are in young adults who received only one dose of MMR vaccine during childhood.

Several provinces, including Nova Scotia, Alberta, New Brunswick and Prince Edward Island, have launched catch-up campaigns to offer a second dose of MMR vaccine to university students or those in Grade 12 and to health-care workers.

Nova Scotia mumps hotspot

Nova Scotia was the epicentre of the Canadian outbreak. As of April 1, it has recorded 788 of the nearly 1,300 cases in this country.

Dr. Shelly Sarwal, a medical officer of health with Nova Scotia's Department of Health Promotion and Protection, said the lingering outbreak, which is "pretty much down to a trickle," taught her and her colleagues that vaccine policy can't be fixed in stone.

"One thing we learned with these [immunization] schedules is that you need to be constantly assessing and reassessing what you're doing," she said from Halifax on Wednesday. "Your schedule should never be static."

Seward said if the Iowa mumps outbreak had continued into the fall of 2006, the CDC was planning to study whether offering another dose of MMR vaccine to university-aged people would have helped to stop spread of the disease. The outbreak petered out before that study could be done, but it is an idea that would be revisited if another occurs, she said.

Dr. Arlene King, an expert on vaccine preventable diseases with the Public Health Agency of Canada said officials in this country hope the catch-up vaccine rounds being offered will lessen the risk of future outbreaks by increasing immunity in susceptible young adults.

As for whether the U.S. experience suggests a third dose or a delay in the delivery of the second dose might be needed, King said time would tell.

"I think that we're going to have to see what the impact of our revised approaches to mumps immunization is in Canada," she said, noting the agency recommends at-risk adults — students, members of the Armed Forces, health-care workers — get a second shot of MMR if they have only had one.

"I think that what we will do is we will observe the epidemiology of the disease to see whether or not that approach will minimize transmission."

Seward agreed it is too soon to say whether these outbreaks were anomalies or harbingers of outbreaks to come.

"You have to look at the cost-benefit of changing schedules or adding a routine dose," she said. "We may not see another outbreak like this for 10 or 20 years."

The CDC study was published Thursday in the New England Journal of Medicine.