“We showed other countries that if you do what we did, you too could get rid of measles,” says Walter A. Orenstein, who was the director of the United States Immunization Program at the CDC from 1988 to 2004. Currently, Orenstein leads Emory University’s Program on Vaccine Policy and Development, but for 26 years he was the US vaccine czar, overseeing two presidential initiatives, first under Carter, and then Clinton, to expand the nation’s collective immunity to infectious diseases, chief among them, measles. “Every World Health Organization region now has the goal to eliminate indigenous measles transmission. It was in part the success of the US that helped in triggering that. We demonstrated it was doable in a country as large and populated as the US.”

On March 16, 2000, Orenstein was among the officials that convened a panel of 22 experts to discuss the state of measles in the US. For nearly four decades, since the invention of the first measles vaccine, the national public health agency had been striving toward a singular goal—driving the virus from US borders. Through a combination of policies that required schoolchildren to get vaccinated and made the highly effective shots accessible to the poorest parts of the population, by the late ’90s, measles was sickening only about 100 people per year—less than one case per million Americans. And nearly all of the cases were imported. The special commission’s job was to decide if that was good enough to finally declare measles stamped out.

While the group of doctors, epidemiologists, and public health experts agreed that the virus appeared to no longer be circulating within US borders, they argued over how to label the accomplishment. They couldn’t really say that measles had been eradicated, for practical purposes more than anything. With the rest of the world, minus Cuba, still firmly in the grips of a measles contagion that killed three-quarters of a million people every year, it would have been close to impossible to ensure that travelers and immigrants wouldn’t bring at least a few cases across US borders now and then. They could call it “eliminated,” but many worried that this would be confusing for the general public, and could even instill a false sense of security, potentially unraveling a decades-long vaccination campaign. But in the end, elimination stuck, because people like Orenstein thought it would be useful for the rest of the world to have a measuring stick, a milestone on the way to total eradication. The definition they agreed upon was the absence of endemic measles transmission in a region for greater than 12 months in the presence of a well-performing surveillance system.

The Americas—an area encompassing 35 countries and 12 territories—reached this goal just a few years later. But it wasn’t until 2016 that the WHO declared it the first of six global regions to eliminate measles. The other five committed to accomplishing the same goal by 2020. Now the chances of that happening are looking increasingly grim. In the first four months of 2019, 179 countries reported 168,193 cases of measles, an increase of 117,000 over 2018, according to the WHO. The biggest spikes have hit Madagascar, Ukraine, and the Philippines, where thousands of children have died. These upticks aren’t happening in a vacuum—the Philippines, Ukraine, and Israel account for the majority of cases imported into the US.

The reasons for these recent failures vary from political instability to economic turmoil to vaccine hesitancy, which the WHO cited as one of the top 10 threats to global health in 2019. And in an increasingly connected world, elimination status is fragile, as the US is finding out. It’s a lesson its neighbor to the south, Venezuela, has also learned recently. Racked by a collapsing economy and crumbling health system, vaccination rates sank to a dismal 52 percent in 2018. An ensuing measles epidemic sickened 7,809 and killed 74. It lost its elimination status in June of last year and has yet to claw it back.