Canada eliminated measles in 1998. The U.S. followed two years later, in 2000. There are multiple criteria for losing an official measles elimination status, including the declines in vaccination rates observed in both countries. But the most significant factor is having ongoing measles transmission for a full year.

But that could also set Canada on another collision course with the Trump administration. During his trip to Asia, Mr. Trump signed trade deals that committed Cambodia and Malaysia to buy American liquefied natural gas, though it is unclear how the United States would enforce those pledges.