The measles virus has not circulated in either country since the late 1990s.
When the virus is detected it means it's been imported — by a resident who contracts measles while travelling, or an infected person visiting the country. Then the highly contagious virus can spread among those who are unimmunized.
What is behind the apparent recent increase in measles activity?
The U.S. had more measles cases in January (102) than it had in most entire years between 2001 and 2010. Canada has recently had several large outbreaks too, one involving 678 cases in Quebec in 2011 and another of over 400 last spring in British Columbia.
Why, when there is an effective — and free — vaccine available to all Canadian kids, are children and adults still coming down with a nasty disease that can progress to pneumonia, trigger brain damage or deafness, and even death?
The fundamental answer is that some parents don't vaccinate their children. But what is really driving multi-case outbreaks is clustering, experts suggest.
What's that mean? Let's use the Five Ws to explore what's going on with measles.
Health authorities have a pretty clear picture of the people who refuse the measles vaccine.
A very small number can't safely be inoculated because of problems with their immune systems.
Some parents refuse to vaccinate for religious reasons, including members of fundamentalist protestant churches with ties to several European countries. Last year's B.C. outbreak started when someone from such a church returned from the Netherlands while infected and started a chain of transmission that led to more than 400 cases among families in the Lower Mainland.
Some parents — sometimes called anti-vaxxers — object to some or all vaccinations on philosophical grounds. They don't believe vaccines are safe or necessary and aren't persuaded by the mountains of scientific evidence that support vaccine use.
They're generally well educated, but are distrustful of conventional medicine and the pharmaceutical industry. They tend to believe they can protect their children better using alternative medicine, says Dr. Kumanan Wilson of the Ottawa Hospital, who has done a lot of research into vaccine refusal.
"The unvaccinated children are unvaccinated I would say in the vast majority of cases by choice — by parental choice — and not by negligence," agrees Dr. Gaston De Serres, an infectious diseases epidemiologist with Quebec's provincial public health agency.
Dr. Shelley Deeks of Public Health Ontario says for the 2012-13 school year, 1.5 per cent of seven-year-old kids in the province were not vaccinated against measles for religious or philosophical reasons, and 0.16 per cent were exempted for medical reasons.
But that doesn't mean 98 per cent of Ontario's seven-year-old children were fully vaccinated against measles. Deeks says 88.3 per cent of seven-year-old kids had received the recommended two doses of measles-containing vaccine that year.
Some of the other children probably received only one dose.
So what's the issue with clustering? You can think of it as the Achilles heel of herd immunity.
The term herd immunity means having enough people protected against a disease that it can't spread within a population. Measles is highly contagious and in unvaccinated populations each person with the virus will infect, on average, 16 to 18 others. So high vaccination rates — 95 per cent and up — are needed to block measles transmission and achieve herd immunity.
Canada's coverage is probably close to that figure, says Dr. John Spika of the Public Health Agency of Canada.
But others argue that without a national vaccination registry for all children and adults, officials can only estimate how many Canadians are protected.
But even if the vaccination rate was 95 per cent, the protection isn't evenly spread across the country.
People who don't vaccinate for religious reasons tend to live in clusters around churches or faith-based schools. Within those pockets, vaccination coverage might be very low.
That means herd immunity doesn't exist locally and a measles importation there could spark many cases.
Public health officials typically know where clusters of religious vaccine objectors live. But it's harder to pinpoint the anti-vaxxers.
These folks definitely form virtual clusters online. Wilson says social media was an unfortunate game-changer for the anti-vaccine movement.
"These people who would be ... interspersed normally and may not meet someone else who has the same belief system — and might eventually have given it up — can now find people who reinforce their belief systems," he says.
Sometimes anti-vaccine families cluster spatially as well. Some schools and communities are more tolerant of alternative thinking.
Before the measles vaccine was introduced around 1970, outbreaks generally occurred in late winter and spring.
Now, Spika says importations often happen in periods when families travel, such as over the Christmas holidays or during the March break. Cases come to Canada from places where the virus is spreading, which in recent years has included the Netherlands, France, India, China and the Philippines.
Public health officials point to a 1998 publication in the journal the Lancet as a seminal event in the anti-vaccination movement.
True, there were vaccine opponents before, and all vaccine rejection is not tied to this paper, but it fuelled a massive surge in the ranks of the anti-vaxxers.
The paper proposed a link between the measles vaccine and development of autism. The paper has since been discredited, disproved and expunged from the medical literature. But damage remains.
— Follow @HelenBranswell on Twitter
Also on HuffPost