A play-by-play of how measles outbreaks can spiral out of control

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Measles infections are ticking up and up across multiple provinces, with more cases already this year than all of 2024.

Local public health alerts of new cases come with detailed descriptions of the person's whereabouts before they were diagnosed.

We're told which hours and minutes a passenger spent in airports in Vancouver, Toronto, and Fredericton, how long other patients were in an emergency department north of Toronto or family health clinic in eastern Ontario, and just how long someone visited a sit-down chain restaurant in Quebec's Laurentians.

The announcements raise a couple questions: Why do measles cases seem to be slipping through health-care professionals' fingers — or even going unrecognized by patients or their families?

And, why give such detailed information on where a person went?

The answers lie in how incredibly contagious the measles virus is, coupled with the cornerstone role vaccination plays in stopping spread.

"What those announcements are trying to do is really make people aware so that they can protect ... others around them and watch out for their own symptoms," said Caroline Colijn, an epidemiologist who holds a Canada 150 Research Chair at Simon Fraser University in Burnaby, B.C.

Last month, Colijn and her co-authors published a modelling study on measles to see what happens when the respiratory illness is introduced to a community, and what level of vaccination coverage or previous immunity from infection is needed to prevent outbreaks.

"Because measles is so incredibly infectious, it can spread rapidly, even if most people in a community, 80 per cent, for example, are immunized," Colijn said. "The usual threshold is that you would want 95 per cent of the people in a community to either have been vaccinated or to have had previous measles exposure."

However, vaccination rates have been falling in Canada and elsewhere, and the early symptoms of measles may go unrecognized — adding to the risk of spread.

Measles spreads through the air when someone coughs, sneezes or talks, so even spending a few minutes in that same airspace can pose an infection risk to someone who isn't vaccinated, such as an infant who is too young to receive the shots. A single infected person can infect 90 per cent of their close contacts, if they're unvaccinated. 

The virus that causes measles is also hardy, lingering on surfaces for two hours after an infected individual leaves.

That's why case counts can spiral quickly. A single person, on average, infects 12 to 18 others who are susceptible to measles.