The number of measles cases around the world nearly doubled from 2022 to 2023, researchers say, presenting a challenge to efforts to achieve and maintain elimination status in many countries.

There were 171,153 cases globally in 2022, according to Dr. Patrick O’Connor of the World Health Organization, who presented the research Saturday at the ESCMID Global Congress in Barcelona. Provisional data shows 321,582 cases for 2023 and more than 94,000 so far in 2024, although the number is probably much higher.

Almost half of this year’s cases have been in WHO’s European Region, with the highest incidence in Azerbaijan, Kyrgyzstan and Yemen.

The U.S. has had 128 measles cases reported in 20 jurisdictions this year, as of Friday, according to the U.S. Centers for Disease Control and Prevention. This is the highest number since 2019.

Measles was declared eliminated in the U.S. in 2000, “meaning there is no measles spreading within the country and new cases are only found when someone contracts measles abroad and returns,” the CDC says. However, the rapid rise in cases this year poses a threat to that disease elimination status, the agency says.

Measles is a highly contagious airborne disease. It can cause serious health consequences or death, especially for young and unvaccinated children.

General symptoms may include fever, cough, runny nose, watery eyes and a rash of red spots. About 1 in 5 unvaccinated people in the U.S. who get measles will be hospitalized, according to the CDC. About 1 in every 20 children with measles will develop pneumonia, and others may develop a dangerous swelling in the brain called encephalitis. Up to 3 of every 1,000 children who become infected with measles may die from respiratory and neurologic complications.

It can also lead to “immune amnesia,” a condition that raises people’s risk of other infections for weeks to years.

O’Connor said Saturday that measles vaccination has prevented an estimated 57 million deaths between 2000 and 2022.

In the U.S., the CDC recommends that children get the first dose of the vaccine that protects against measles, mumps, and rubella (MMR) between 12 and 15 months of age. Kids get a second shot between four-and-six years of age.

The vaccine is considered highly effective. One dose is 93-per-cent effective against measles, and two doses are 97-per-cent effective. Vaccinated people can still get sick, but it doesn’t happen often, and typically, it’s a milder infection.

The U.S. has set a target vaccination rate of 95 per cent, but coverage among kindergarteners has dipped below that in recent years. In the 2022-23 school year, just 93.1 per cent of kindergarteners in the U.S. had completed their MMR vaccine series, leaving about 250,000 at risk.

Measles is “a crisis among many crises,” O’Connor said in his presentation Saturday, with about 45 per cent of outbreaks in conflict-affected and fragile countries.

“Over the last 20 years, there has been significant progress toward achieving measles and rubella elimination,” he said in a news release. “In order to solidify and maintain those gains, we need to ensure high, uniform and equitable routine immunization coverage; and robust outreach and rapid outbreak response.”