Measles hasn’t been endemic to the United States since 2000. It’s a remarkable example of effective public health, education and science.

Now, that may be changing. The Centers for Disease Control and Prevention announced on May 30 that the country is in danger of losing its measles elimination status. It’s currently experiencing its highest levels of the disease since 1992, with more than 1,000 cases reported so far this year. In other words, measles is in danger of becoming endemic in the United States again.

The next few months will be critical, said Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine. Thus far, outbreaks have been confined to individual pockets scattered around the country. Over the next few months, scientists will be watching to see whether transmission begins to occur between the pockets.

But in any case, “the fact that we’ve potentially allowed it to come back is profoundly demoralizing,” Hotez said.

Why Elimination Status Matters

Losing an official status may seem more symbolic than practical. But experts say the change could have serious consequences for the future of measles in the country. It could even hurt vaccination campaigns in countries where measles is a significant cause of death. A full-on global effort to vaccinate children cut the number of measles deaths worldwide by 80% from the 545,000 deaths in 2000. But that still means more than 100,000 people, mostly children younger than five, died from the disease in 2017.

Because of the United States’ leadership role – it was the first country to eliminate measles – a change in status may discourage public health officials from focusing valuable resources on immunization efforts.

How the U.S. Got Here

The elimination of measles in the United States is a remarkable success story.

Before the 1960s, nearly every child in the country fell sick with the disease before adolescence. Many developed complications, including ear infections, pneumonia, even encephalitis. And every year, approximately 400 to 500 people in the States died of measles-related complications.

An effective vaccine was developed in the 1960s, and rates immediately began to decline. Beginning in the late 1970s, in a widely supported public campaign, the CDC set out to eliminate measles from the country entirely.

It worked. In 2000, measles was declared to be officially eradicated from the country. While people occasionally brought the disease back from international travel, widespread vaccination prevented the disease from gaining a foothold.

The U.S. may have done too good a job. “The vaccine has been a victim of its own success,” said Paul Offit, a professor of vaccinology at the University of Pennsylvania and co-developer of the rotavirus vaccine. “We’ve not only eliminated disease, we’ve eliminated the memory of disease.”

The loss of that memory – of miserably sick children and hundreds of yearly deaths in the United States – means that parents persuaded by vaccine hesitancy don’t know what they’re up against. Among other techniques, anti-vaccination activists argue that measles isn’t a severe enough disease to be worth the risk of vaccination.

That’s simply untrue, according to William Schaffner, a professor who studies infectious disease and immunization at Vanderbilt University School of Medicine. “In many parts of the developing world, [measles] remains a substantial killer of children,” he said.

And in ways, more sophisticated medical care in the United States has made measles more of a threat, Schaffner said. Cancers and other serious health issues aren’t always the death sentence for children they once were. That means we have many more children who are alive but struggling with weakened immune systems. They’re not well enough to be vaccinated, but they’re more vulnerable to the very diseases that vaccinations exist to prevent.

What Happens Next

According to Hotez, the summer could see an easing off of measles cases – or new outbreaks. On the one hand, measles epidemics have historically peaked in the spring and ebbed in the summer, which means the U.S. is heading into a measles off-season. But summer brings a lot of international travel, which is the primary way that measles is introduced here. If travelers bring measles back with them to communities with low vaccination rates, the disease could begin to spread more widely. “We’ll have to see which force wins out,” Hotez said.

Preventing transmission will be an important part of the process. Direct Relief stocks Gamastan, an effective post-exposure medication, in case of measles outbreaks. The organization recently sent supplies to Butte County, where cases were confirmed.

The world is currently experiencing several major measles outbreaks, each with its own complex causes. Two factors that are most common: lack of access, when families can’t get to a clinic or afford the vaccine; and vaccine hesitancy.

We export our culture, our music, our movies. Now we’re exporting our anti-vaccine movement,” Hotez said.

“We were the models” for measles elimination, said Schaffner. The vaccine was an American invention, and the elimination of measles an American success story. “I think the concern in public health is that other countries may see what’s happening and say the juice isn’t worth the squeeze.”

In other words, if the United States’ measles effort ultimately fails, other governments might be hesitant to pour money, time, and energy into their own vaccination campaigns.

What’s Different This Time

So once again, public health leaders find themselves trying to stop the spread of measles. But it’s different this time around. The outbreaks are smaller and more contained, thanks to generally high vaccination rates across the country. However, public trust, vital to the success of the 20th century’s measles elimination campaign, is lacking. Misinformation is rife. “We’re much more cynical than we were back then,” Offit said.

Offit divides people who don’t vaccinate into two groups. In the first, larger group are parents who have heard the arguments against vaccines and are concerned, but not necessarily convinced. If they’re given reliable information in a “passionate, compassionate, compelling way,” they can generally be persuaded.

But for people who are completely convinced by anti-vaccine rhetoric, Offit’s experience tells him that education won’t work. That’s where public health legislation can step in, through the elimination of personal-belief and religious exemptions or even, in the case of New York’s Rockland County, banning unvaccinated children from public spaces.

In Offit’s eyes, the country no longer deserves its elimination status. “We can say right now that measles is endemic in the United States,” he said. The numbers are “a blip” compared to what they were, but statistically speaking, one in every thousand measles cases results in a death. “We’re on the verge of seeing people die of measles again,” he said.