TORONTO -- A large measles outbreak that occurred in Quebec a couple of years ago is raising more questions about whether babies are getting their first dose of measles vaccine at too young an age.
A new analysis of the 2011 outbreak, which involved a surprising number of children who should have been protected against the virus, suggests giving the first dose of measles vaccine at 12 months may be undermining the efficacy of the vaccination program.
And it raises the possibility a growing pool of vaccinated children may still be vulnerable to infection -- which, if true, could put in jeopardy efforts to eliminate and eventually eradicate measles, one of the authors of the study said.
"This study shows that there is vulnerability in children who receive two doses and we should not overlook that. The reason for that is still unknown. But it is important to dig into this question further," said Dr. Gaston de Serres, an infectious diseases specialist with Quebec's provincial public health agency.
"If we create a vast pool of susceptibles, one day or another it won't be an outbreak of 700 cases like we had in Quebec, but it may be thousands of cases, if not more."
The new study was published today in the journal Pediatrics.
De Serres and colleagues have been mining data from the 2011 high school outbreak to try to figure out why 102 of the 725 children infected caught the disease at all. They had received the recommended two doses of measles vaccine when they were young children.
An earlier study by the same researchers showed that in the outbreak, teens who got their first dose at 12 months -- the time recommended across Canada -- were six times more likely to be infected than those who got their first shot at 15 months of age.
When they published that study, one of the suggestions was that a phenomenon known as "maternal antibody" may have been responsible.
It takes time for the human immune system to develop. But babies survive infancy because they receive antibodies to diseases their mothers have either experienced through infection or been introduced to through vaccination.
Maternal antibodies wane over time. But while they are present, if a baby is given a vaccine made from a live but weakened virus, the maternal antibodies may quell the vaccine virus before it has a chance to induce a strong immune response. Measles vaccine is a live-virus serum.
So immunization programs are planned to try to hit the sweet spot, giving vaccines in time to protect toddlers against threats but not so soon the vaccines won't be fully effective.
It's thought women who had measles would have higher levels of maternal antibodies than women whose immunity was due to vaccination. Most people born before measles vaccination programs began in Canada in the early 1970s would have been infected with the highly contagious virus.
-- The Canadian Press