Why measles is returning to the Americas

The elimination of measles from the Americas in 2016 was spectacular. No other region of the world had ever managed to go a year without sustained transmission of the disease, the UN World Health Organisation’s bar for elimination. But it did not last long. In 2017 cases flared in patchily vaccinated Venezuela. Migrants fleeing hyperinflation and regime brutality carried measles over the border to Brazil. It tore through the deprived Amazon area before finding its way to the densely populated state of São Paulo. By the time covid-19 arrived three years later, with lockdowns and masks curbing transmission, measles had infected at least 30,000 people and killed more than a hundred, mostly young children or infants. It was the worst period for measles in the Americas for 22 years.

The current outbreak is set to be worse (see chart 1). By April 25th the Pan American Health Organisation (PAHO), an arm of the WHO, had recorded 18,352 confirmed cases across the Americas in 2026. The virus is spreading, having infected 14,503 people last year. At least 45 people have died in the outbreak so far, mostly children. The majority of cases are in Mexico and Guatemala, but the virus has got a foothold in Bolivia and Peru too.

Measles is still nowhere near as prevalent in the Americas as it was in the 20th century, before vaccination efforts began; 200,000 cases a year was the norm. Hantavirus, a pathogen found in rodents that is prevalent in South America, is the disease in the news recently after it infected people on a cruise in South America. But measles is far more infectious, and far more dangerous if not contained with vaccination programmes. Rolling outbreaks suggest efforts to do so are faltering. Most vulnerable are babies before their first jab, who risk brain damage, blindness or death.

The latest outbreak started at a wedding in Canada in October 2024, attended by Mennonites, a close-knit Christian group who often eschew vaccination. One guest travelled from Thailand carrying measles. Others were infected, and carried the disease back to their homes and communities. By early 2025 the virus had spread to Mennonites in Texas and Mexico. “It is unprecedented how [its spread] has been related to a socially isolated group with connections throughout the hemisphere, from Argentina up through Canada,” says Jon Andrus of Paho.

In Canada and the United States the outbreak has been largely confined to Mennonites. But in Mexico and other poorer places in the Americas, slipping vaccination programmes have exposed the general population to infection. In 2014 some 96% of Mexican children received a second dose of measles vaccine, above the 95% required for a population to benefit from herd immunity. By 2024 that number was less than 70%. Many other Latin American countries moved in the same direction over that period (see chart 2). Latin America has gone from having one of the world’s highest rates of childhood vaccination to one of the lowest.

The outbreak prompted Mexico’s government to start an emergency immunisation drive. Between January 2025 and March 2026 it gave out some 30m doses. Nurses have been going door to door offering the jab, and setting up vaccine centres in shops and at bus stations. Cases appear to have peaked, for now, says Dr Andrus, but a new challenge looms. On June 11th Mexico will begin hosting the football World Cup, together with the United States and Canada. FIFA projects that some 5.5m fans will attend the tournament. PAHO is urging host countries to start actively looking for measles infections so as to detect any new cases promptly and curb the spread of the disease.

While cases may have peaked in Mexico, they are still rising in Guatemala. Some 5,399 people have been infected, with at least four children killed. The outbreak began late last year at a religious gathering of more than 2,000 people which drew visitors from the United States, Mexico and Central America. More large religious events are planned. The health ministry is concerned. It is organising workshops to educate people about vaccines.

The backsliding which is allowing measles to spread through the Americas once more has several causes. Covid-19 damaged health-care services across the region, pulling staff off vaccination programmes. Populist leaders in Mexico, Argentina and Brazil have all cut their budgets for vaccinating children in recent years. Rising vaccine hesitancy, driven in part by unfounded concerns spread on social media about the safety of jabs, plays a larger role in rich countries. In Canada the childhood vaccination rate has dropped from 86% to 79% over the past decade. “I am terribly worried,” says Dr Andrus.

Against this weakening wall of immunity, the virus arrives constantly from places where measles is still rife, creating the risk of an outbreak. The lower the vaccination rate, the more likely the virus is to spread. When rates in the Americas were high, groups like Mennonites enjoyed the protection of the herd. As overall rates have fallen, groups with particularly low rates endanger themselves and others.

What can be done? In the midst of an outbreak, contact tracing can prevent further spread, while vaccination drives often get a high uptake from worried parents. In the longer run, Daniel Salas of PAHO says modernising immunisation infrastructure is important. Today only 19 of the 35 countries PAHO covers have some form of electronic immunisation registry that can alert parents when their children are due for vaccines. Outreach using culturally sensitive and faith-based vaccination instructors may prove most effective.

Political will is critical. The Americas’ role model is Uruguay, the first country in Latin America that had a vaccine register. “They have good surveillance, and they have a political commitment that has never wavered from prioritising prevention of preventable diseases,” says Dr Andrus. Argentina, where vaccination rates have dropped precipitously under the austerity policies of Javier Milei, risks an explosive outbreak without a timely and determined effort. Measles, says Dr Andrus, does not allow countries to drop their guard. ■