It's an inescapable reality: As long as measles is infecting children in other parts of the world, Canada is going to

have occasional outbreaks. The same is true in the United States, but public health officials there typically have managed to more quickly extinguish spread of the virus when it comes from abroad.

It's leaving some experts on this side of the border wondering: Why does Canada have such large measles outbreaks?

There have been at least 375 cases in the ongoing outbreak in British Columbia's Fraser Valley. An outbreak in Quebec in 2011 racked up a whopping 725 confirmed cases, following on a 2007 outbreak of 94 cases in that province. The Vancouver Olympics, which brought people to the city from around the world, touched off an outbreak of roughly 80 cases in B.C. in early 2010.

By comparison the largest outbreak the U.S. has experienced in nearly two decades occurred in an Orthodox Jewish community in Brooklyn, N.Y., last year. Total cases: 58. Indeed most measles outbreaks south of the border are far smaller, coming in at under two dozen cases, and many times fewer than a handful.

What explains the substantial difference in numbers?

"Well, that's the question," Dr. Gaston De Serres says. "That's the question."

"They have more importations per year than Canada. And yet, you know, the size of outbreaks they had in the U.S. in the past 10 years is way smaller than what we have in Canada."

De Serres is an infectious diseases specialist with Quebec's provincial public health agency; he led intensive studies into that province's 2011 outbreak. He and some colleagues are now looking at this Canada-U.S. question, trying to figure out how to explain why Canada has had such large outbreaks of late.

Their investigation will likely lead to a scientific paper, so De Serres is reluctant to spell out all his theories now. But one

possibility he raises relates to the way public health authorities north and south of the border respond to reports of an imported case of measles.

"The Americans are far, far more aggressive in terms of tracking every case and implementing control measures," says De Serres.

"That might be a difference between the countries."

Both countries have high measles vaccination coverage — coverage that is high enough to have stopped measles viruses from spreading here regularly, as they did before the introduction of the vaccine. In fact, the virus is no longer endemic in the countries of the Americas, which are in the process of trying to be certified as having eliminated measles.

Even 1 measles case acted on like an outbreak

Measles is a respiratory disease caused by a virus of the same name. It causes fever, runny nose and a characteristic rash all over the body. Most people recover, but the infection is fatal in between one and three of every 1,000 cases.

Measles is targeted for global eradication. When all six of the World Health Organization regions stop local spread of measles, the disease can be declared eradicated.

The Pan American Health Organization, the WHO regional body for this region, said the Americas achieved elimination status in 2002, but the process of formally certifying that accomplishment hasn't yet been completed. And it could conceivably be threatened by large outbreaks sparked by imported measles cases, such as Quebec's 2011

outbreak. That outbreak lasted almost a year; 12 months of continuous spread of measles in one country would reset the clock on the elimination effort for the entire region.

The Public Health Agency of Canada describes measles vaccination coverage in Canada high but uneven. A 2009 national immunization survey found about 92 per cent of two-year-olds had received the first of two doses, but only 69 per cent of seven-year-olds had received the second dose. Rates are similar in the U.S., where in 2012 nearly 91 per cent of children had received at least one dose of measles vaccine.

Both countries have pockets of unvaccinated children. When those pockets are small — a few kids in a school full of vaccinated children — a case of measles brought back from a country where the virus circulates won't trigger a big outbreak because there are few unprotected kids to infect.

But when there are more unvaccinated children — or adults — transmission can go on for weeks, as it has in the Fraser Valley outbreak. There spread has emanated from a religious school — kindergarten to Grade 12 — affiliated with a church that opposes vaccination.

Sill, De Serres says Canada does not have more vaccine opponents than the U.S. does, and both countries have religious groups that oppose vaccination. So if the circumstances are roughly the same, is the response what makes the difference?

A 2008 outbreak in San Diego, Calif., is a good illustration of the lengths to which public health officials in the United States go to contain measles when the virus makes an incursion.

An unvaccinated seven-year-old child contracted measles in Switzerland, and infected 11 others on returning home. Public health swooped in, tracing contacts of all cases who were exposed, sending home unvaccinated children and quarantining families. Unvaccinated children cannot return to school until 21 days after the last case developed symptoms, which can mean sitting out several months. They can go back to school immediately if their parents have them vaccinated.

A report on the outbreak, published in the journal Pediatrics, said county and state public health workers logged 1,745

person-hours on investigation and containment efforts, at a cost of $10,376 US per case. Affected families reported direct and indirect costs of $775 US per quarantined child.

Dr. Jane Seward is the top measles expert at the U.S. Centers for Disease Control in Atlanta. She says the response to measles across the United States is swift and robust.

"Even one case in the United States is considered an 'outbreak.' Although our outbreak definition is three or more cases, with even a single case our surveillance guidelines are for states to very, very rapidly report that case," she says.

"Even a suspected case before confirmation, it needs to be reported to the CDC within 24 hours. And then the case needs to be investigated and all the people who were in contact with that case need to be assessed for evidence of immunity and offered vaccine or immune globulin" — a serum of antibodies — "as appropriate if they don't have evidence of immunity."

People who are sick are told to isolate themselves. People who are exposed but don't yet have symptoms are told to quarantine themselves. People who don't voluntarily comply are sometimes hit with court orders to do so, such as in a 2004 measles outbreak in Iowa.

This aggressive approach is not cheap. A study by CDC scientists published in the journal Vaccine reported that 16 measles outbreaks in the U.S. in 2011 cost taxpayers between $2.7 million US and $5.3 million US.

Canadian public health authorities have essentially the same tools at their disposal but may not employ them as rigorously as their American counterparts.

Dr. Lisa Mu, medical officer of health with the Fraser Health Authority, says the B.C. Public Health Act gives public health workers broad powers, but authorities prefer not to use them if they don't have to. Fraser Health tries to take a collaborative approach, Mu says.

"We want to maintain our healthy relationships with the public, and not be seen as enforcers," she says.

In the ongoing outbreak there — which has placed a huge burden on the public health staff who too have traced contacts of cases — Mu says most people who have been asked to go into isolation or quarantine have obliged. But health authorities have received reports some people who should be home have continued to move about the community, behaviour that could potentially expose others to measles.

Fraser Health has not taken action. Mu says the decision comes down to how high the risk is and how certain it is that the behaviour will threaten others.

"When we have possibly hundreds of people who are being requested to remain at home because they may have come in contact with somebody who may have had measles, or they were in contact with somebody who may have had measles, the risk there is not grave enough, the certainty is not there for us to exercise those kinds of limitations on somebody's personal freedoms," she says.

"So to date we have not enforced any kind of isolation orders or quarantine orders."

The school that is at the heart of the outbreak was closed for an extended spring break, she says. It has now reopened.