A second study from Quebec is calling into question the timing at which young children are vaccinated against measles.
The new research shows that teenagers who received the recommended two doses of measles vaccine but who got the first shot when they were 12 months old were six times more likely to go on to contract the disease than those who got their first dose at 15 months.
The work was done to try to discover why a number of teenagers who would have been assumed to have been protected - because they got two doses of vaccine in childhood - nevertheless were infected during Quebec's large measles outbreak in 2011.
More than 700 measles cases were reported in that outbreak.
Lead author Dr. Gaston De Serres of Quebec's provincial public health agency presented the work recently at a major international infectious diseases conference in San Francisco. The study was a followup to an initial paper on the outbreak that he presented to the same conference last year.
While the situation bears watching, De Serres said Canadian policy on timing of measles vaccine delivery isn't likely to change just yet.
"For sa国际传媒, I would probably say that at this time we will not change the schedule," De Serres said from the Interscience Conference on Antimicrobial Agents and Chemotherapy.
"We need to follow up with more studies."
Prior to the measles vaccine, most children contracted the disease and about two million a year died from it.
One of the most contagious diseases to afflict humans, measles has largely been quelled in the Americas, though it remains endemic in other parts of the world.
Still, outbreaks do crop up in North America.
Generally, those outbreaks take place among children whose parents refused to vaccinate them. It is rare to see cases in vaccinated children.
While measles can be particularly dangerous for infants, they cannot be vaccinated. That's because antibodies they get from their mothers while they were in the womb kill the virus in the vaccine before it has a chance to induce an immune response in the child.
That phenomenon is called maternal antibody interference. Maternal antibodies protect an infant while its own immune system is just starting to develop. The effect of maternal antibodies wanes in the months after birth.
The World Health Organization recommends giving the first dose of measles vaccine at nine months, with the second dose to follow any time after 12 months.
Jane Seward, a measles expert with the U.S. Centers for Disease Control, said the goal is to vaccinate children as soon as the vaccine can take effect so as to minimize the risk of death and severe disease.
"It's a balance between getting the first dose as early as possible to decrease deaths and as late as possible to get the best immune response," she said.
