Why the US, Mexico and Guatemala may lose their measles-free status

International health experts are meeting in Washington, DC, this week to determine which countries in the Americas have endemic measles transmission.

The outcome could decide whether the United States, Mexico and Guatemala retain their “measles elimination status”: a designation that the virus has not spread uncontrolled domestically for at least 12 months.

The Pan American Health Organization’s (PAHO) Regional Monitoring and Re-Verification Commission (RVC) began its annual conference on Tuesday to assess the countries’ status. The conference will adjourn on Friday.

The review comes as a sharp resurgence of the virus has spread across the Americas, with the US experiencing its largest and deadliest measles outbreak in nearly 30 years. The US has held its elimination status since 2000, but it risks losing it this year.

The commission will review reports submitted by all 35 PAHO member states, with particular attention to countries experiencing prolonged outbreaks.

How does the process work? Al Jazeera breaks down what happens in this explainer.

Who decides whether a country has eliminated measles?

The RVC is an independent panel of international experts in epidemiology, public health and virology. The group advises PAHO, which is part of the World Health Organization, on which countries have maintained the elimination of measles and rubella.

The Americas became the first region in the world to eliminate endemic measles transmission in 2016, following decades of vaccination campaigns. For this year’s review, the commission is examining epidemiological data from 2025 and the first half of 2026.

Which countries could lose their elimination status this year?

The US, Mexico, Guatemala, Peru and Bolivia are at risk. Canada lost its status last November.

The US, which eliminated endemic measles transmission in 2000, has experienced sustained outbreaks since January 2025. Mexico has also reported prolonged transmission.

PAHO reported 57,666 confirmed measles cases across 17 countries and territories in the Americas between January and October 2026.

How does a country lose its status?

Experts at the conference evaluate whether infections result from separate introductions of the virus or whether the same strain has circulated locally.

Reported measles cases or deaths alone do not determine whether a country loses its elimination status. Some cases, for instance, come from exposure to measles during travel. A limited outbreak due to such exposure would not result in the loss of a country’s measles elimination status.

Under PAHO’s criteria, a country loses its elimination status if the same measles virus strain continues to spread uninterrupted for at least 12 months in a country or defined geographical area.

Investigators use genetic sequencing to determine whether infections are linked to the same virus strain. They also examine epidemiological data to establish how the virus has spread and whether transmission has been interrupted.

In addition, the commission considers vaccination coverage, how healthcare systems monitor measles cases, and the speed of outbreak responses in its decisions.

When will the decision be announced?

The commission’s deliberations in Washington run until Friday. It will assess the evidence and make recommendations on the status of individual countries to PAHO Director Jarbas Barbosa, who will make the final determination.

The official announcement is expected early next week following the conclusion of the conference.

What happens if the US loses its status?

Experts agree that the US will likely lose its measles elimination status, marking a reversal of a major public-health achievement.

“It would be a disappointment, not a shock,” Matt Ferrari, the director of Pennsylvania State University’s Center for Infectious Disease Dynamics, told Al Jazeera. “This is moving backwards from a great public health achievement, and to see that backslide is just heart-wrenching.”

Although the loss of status would not trigger any immediate consequences, those working in the field say it would be an embarrassment.

“It’s a stain on your health programme,” Edwin Asturias, a professor of infectious disease and epidemiology at the University of Colorado, told Al Jazeera.

The long-term effects include greater pressure on public-health systems, which must devote resources to identifying infections, tracing contacts, conducting laboratory tests, isolating patients and organising vaccination campaigns.

When asked about how a country generally responds to losing its measles elimination status, Asturias pointed to Brazil, which found itself in that predicament in 2019. The country regained its measles-free designation in 2024 after launching a strategic vaccination campaign. But Asturias does not believe that the US will mount a similarly robust response.

On Tuesday, Erica Schwartz, the director of the US Centers for Disease Control and Prevention, said losing elimination status would not change the agency’s approach to measles.

“Let’s say we did lose our measles elimination status. We’re going to be doing the same exact thing that we’ve been doing today, which is working with the states,” Schwartz told the news agency Reuters.

Ferrari said the announcement could nonetheless galvanise action at state and local levels by giving public-health officials and advocates shared language.

“Think globally, act locally,” Ferrari said. “We all will have the same language to talk about what’s going on. And that’s surprisingly important.”