Hello,

On Monday, the Pan American Health Organization — a group that comprises about 35 countries in the Caribbean and North, South, and Central America — revoked Canada’s measles-free status. 

Next year, the group will determine whether the United States also will lose its status as a country that eliminated measles. PAHO defines this status as the absence of regular, endemic transmission. In other words, small outbreaks sparked by people infected abroad don’t spread for long. 

This isn’t a simple distinction because the origin of measles outbreaks is rarely obvious. For example, the West Texas emergency sparked outbreaks in New Mexico, Kansas, and Oklahoma — at the very least — but the trigger for others across the country is unknown.

I spoke with Daniel Salas, the executive manager of PAHO’s immunization program, about how the group makes its determinations. 

These excerpts from our conversation have been edited for length and clarity.   

Most measles viruses circulating in Canada appear to be the same strain, but it’s a common variety throughout the Americas. So how can you tell if the outbreaks are linked?

Authorities in Canada didn’t have all of the epidemiological information to show links. It’s really difficult to trace all of the contacts in these outbreaks. So the authorities decided that since they couldn’t bring specific information linking or unlinking cases, and since it was all basically the same genotype, the same strain, they would attribute it to internal transmission.  

Hypothetically speaking, what if U.S. authorities have a similar lack of data but make the opposite decision, saying that the outbreaks have been imported and, therefore, the U.S. shouldn’t lose its status?

We expect countries to be transparent about the information they have. The RVC [PAHO’s regional verification commission] asks questions after authorities present their reports. How did you make your determination? Did you consider other angles to the situation? Did you consider this or that? These are the exchanges we have at our meetings. 

Then they make an assessment on whether to change the status of a country. They submit their recommendations to the PAHO director, and the PAHO director makes a decision.

Is the U.S. still talking with PAHO, given President Donald Trump’s withdrawal from other international groups?

Yes, the CDC was in the RVC meeting last week, and they were really listening to our recommendations. 

PAHO can reverse its decision on measles elimination status after a year. Is it possible that this year has just been a very bad one for measles in Canada and the U.S. — and then we’ll get back to normal?

It’s possible because the outbreaks have mainly been concentrated in close-knit communities of susceptibles. By susceptibles, I mean people who are under-vaccinated or not vaccinated. In Canada, the outbreaks spread quickly in these groups, but they haven’t spilled out to the general population because of good vaccine coverage. We have seen some cases in the general population, but not enough for sustained transmission in communities. 

The number of cases in Canada has been dropping, and I expect these outbreaks to come to an end unless the virus finds a new pocket of unvaccinated people. 

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