Measles vaccine less effective if given too early, study finds

Canadian babies are being immunized for the measles at an age that may not be optimal for protecting them from the deadly disease, according to new research.

A study being published Monday in the journal Pediatrics found that a difference of a few months can alter how well a child is protected from measles later on in life.

In the study, researchers looked at data from the 2011 measles outbreak that occurred in Quebec to determine why so many students were infected. In total, more than 700 students contracted the illness, with the majority of patients between 10 and 19 years old.

Many of the infected students were not vaccinated, said Dr. Gaston De Serres, medical epidemiologist at Quebec's institute of public health and one of the study's authors. But the researchers wanted to determine whether there were other factors at play.

While vaccination rates played the largest role, they also discovered that vaccinated children were also more vulnerable if they received their first measles shot at 12 or 13 months of age rather than 15 months or older. Current recommendations call for children to receive two measles vaccines. In Canada, the first is recommended at 12 months while the second can be given months later. Evidence shows that two vaccines confer greater protection.

When researchers looked at a school where the outbreak was centred, they found that about 5 per cent of students were not vaccinated, a number that is far too high, De Serres said. Of those, more than 80 per cent came down with measles.

But among students who received two doses of the measles vaccine, 3.5 per cent also came down with the illness. Children who received the vaccine at 12 months were three to four times more likely to get infected than students who had their first shot 15 months of age or older.

De Serres explained that mothers transmit antibodies, which fight off infection, to their children when they are born. Those antibodies could last for months. That means if a vaccine is given when those antibodies are still active in the infant's bloodstream, they could damage the vaccine, diminishing its effectiveness.

The study could call the current vaccine recommendations into question and may suggest that all Canadian children be vaccinated at age 15 months and older, De Serres said.