By Helen Branswell, The Canadian Press
TORONTO – 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.
But an unusual observation from a large outbreak in Quebec may raise some alarm among those who attend the conference, the annual meeting of the Infectious Diseases Society of America.
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.