An unusual vaccine observation from a large measles outbreak in Quebec may raise some alarm among those who attend the annual meeting of the Infectious Diseases Society of America.

A still smoldering outbreak of measles in Quebec is the largest in the Americas in over a decade. (U.S. Centers for Disease Control and Prevention/Canadian Press) Measles cases have surged in parts of Canada and the United States this year, with cases among unvaccinated children and teens driving the high numbers, public health officials from both countries will tell a major infectious diseases conference this weekend.

An investigation into an outbreak in a high school in a town that was heavily hit by the virus found that about half of the cases were in teens who had received the recommended two doses of vaccine in childhood — in other words, teens whom authorities would have expected to have been protected from the measles virus.

It's generally assumed that the measles vaccine, when given in a two-dose schedule in early childhood, should protect against measles infection about 99 per cent of the time. So the discovery that 52 of the 98 teens who caught measles were fully vaccinated came as a shock to the researchers who conducted the investigation.

"That's the real question. How could that have happened?" said Dr. Gaston De Serres, an infectious diseases expert with Quebec's public health agency and one of the authors of the study.

In an interview before the start of the conference, De Serres would not name the highly affected town or the high school in it.

But he suggested the discovery that as many of the cases were fully vaccinated as unvaccinated raises a serious question about whether the timing of the delivery of the first dose of measles vaccine is undermining the efficacy of the prevention program.

Vaccine schedule questions

It is recommended that the first dose of measles vaccine be administered at 12 months of age, with the second coming at either 18 months or some time before a child starts school. Quebec uses the 12-month and 18-month schedule.

The vaccine can't be given earlier, because of a phenomenon that helps babies survive infancy. Children are born without a fully developed immune system — it starts to build as babies become exposed to a variety of disease threats over their first few years.

In pregnancy and after birth, through breastfeeding, babies acquire antibodies from their mothers that tide them over until they can make their own. But that means if they are given the measles vaccine — which is made from weakened live viruses — too early, their mothers' antibodies will kill the vaccine viruses, preventing protection from being induced.

It has been thought that 12 months was appropriately late for vaccination to begin, and it may still be. De Serres insisted other studies will have to confirm his group's findings before anyone starts advocating delaying the first dose of measles vaccine.

"This is the first outbreak where an investigation has shown this effect in two-dose recipients," he said.

But the study did note that teens in the school who got their first dose of measles vaccine at 15 months of age were more than three times less likely to get measles in the outbreak than teens who got their first shot at 12 months.

If other groups confirm what the Quebec investigation found, it could mean there is a lot more susceptibility to measles in the vaccinated population than is currently being assumed, De Serres admitted.

"There may be more vulnerability than we know and were planning for," he said.

Coming back with measles

"The problem is: if you have a population where the disease does not enter often, you may have a pool of susceptibles that is growing up, year after year, and you will be unaware of that vulnerability."

Changing vaccine schedules is not something that is done lightly, especially at a time when little children are getting an increasing number of vaccinations.

And there's another factor one would have to consider when debating the question, noted Dr. Allison McGeer, an infectious diseases expert at Toronto's Mount Sinai Hospital.

Many measles cases seen in Canada are in the children of new Canadians who have travelled to their homelands to show off their new babies. Some of those families are travelling to places where measles viruses are still circulating.

"If you knew that a child was going to stay in Canada … a 15-month first dose [schedule], based on this experience, certainly sounds like a sensible thing to do," McGeer said.

"But measles is one of those diseases where when immigrant parents take their children home to see the grandparents, it frequently happens before or just about the age of one year. … If you shift the first dose to older, some of those kids will come back with measles."

After the Boston meeting De Serres will be travelling to Atlanta to brief the U.S. Advisory Committee on Immunization Practices of his group's findings. ACIP set vaccine policy for the U.S.