Measles is on the rise around the world, and even experts who saw it coming say the increase is "staggering."

The World Health Organization said in December that its European region (which extends into parts of western and central Asia) saw an "alarming" increase in measles cases – from under a thousand in 2022 to more than 30,000 last year.

John Vertefeuille, director of the U.S. Centers for Disease Control and Prevention's Global Immunization Division, said in a statement that the numbers are "staggering."

The WHO's most recent global numbers, released in November, reveal that measles cases increased worldwide by 18% to about 9 million, and deaths rose 43% to 136,000, in 2022 compared to 2021. Some 32 countries had large, disruptive outbreaks in 2022, and that number ticked up to 51 in 2023, Dr. Natasha Crowcroft, WHO's senior technical adviser for measles and rubella control, told NPR.

The worrying uptick in measles outbreaks and deaths is, "unfortunately, not unexpected given the declining vaccination rates we've seen in the past few years," noted John Vertefeuille of the CDC in his statement. "Urgent, targeted efforts are critical to prevent measles disease and deaths."

Measles is one of the most contagious infectious diseases, and also one of the most preventable: two doses of vaccine in childhood is 97% protective. WHO estimates that some 61 million doses were missed or delayed in 2021. In 2022, about 83% of the world's children received one dose of measles vaccine by their first birthday – the lowest proportion since 2008, when the rate was also 83%.

"We're going to see outbreaks any time we have an accumulation of people who haven't been vaccinated," says Cyndi Hatcher, unit lead for measles elimination in the African Region at the CDC. "When you have immunization disruptions, measles is always going to be one of the first epidemics that you see."

Low-income countries continue to have the lowest vaccination rates – five sub-Saharan African countries have rates below 50% for the first dose.

"Measles is called the inequity virus for good reason. It is the disease that will find and attack those who aren't protected," says Dr. Kate O'Brien, WHO director for immunization, vaccine and biologicals.

In Ethiopia, for example, conflict and weaknesses in the rural health system have taken a toll on vaccination rates, says Dr. Ngozi Kennedy, UNICEF's Ethiopia health manager.

"We have a lot of pastoral communities that are often on the move so they may not know how to, or may not be able to, get to health centers for the vaccine. Also, as a result of the protracted conflicts, services are often disrupted with populations and even some health-care workers being displaced," she says.

Children who don't get their vaccines on schedule are at risk of death and serious illness, particularly children under age 5 who are at highest risk for severe complications including pneumonia, encephalitis (brain swelling) and death. Measles can also put children at higher risk for other potentially fatal childhood diseases – such as diarrheal diseases and meningitis – because the virus can cause the immune system to forget its learned defenses against other pathogens.

"I think that people may have forgotten how dangerous measles can be if they haven't seen cases before," Hatcher says.

But global health experts didn't forget, and many predicted that outbreaks would be coming.

"During the pandemic, when everything was locked down, there wasn't much measles being spread ... because no one was going anywhere," says WHO measles and rubella senior technical adviser Dr. Natasha Crowcroft. "It's the usual human thing that no one does anything until the problem starts. It's really hard to sell prevention."

Last year a coalition including the Bill & Melinda Gates Foundation and Gavi, the Vaccine Alliance, launched "The Big Catch-Up" – an effort to get vaccination rates back up to at least their pre-pandemic levels. (Editor's note: The Gates Foundation is one of the funders of NPR and this blog.)

Kennedy says efforts continue in Ethiopia to shore up vaccination rates. Health workers there have begun to track childhood immunizations electronically in hopes of keeping more children current, and the country has prioritized 14 "equity zones" to catch kids up on their shots.

But the CDC's Cyndi Hatcher says much more needs to be done.

"I think we need to be very honest with ourselves at the global level," she says. "Are we truly committed to making [measles] a public health priority and do we have the resources that we need to make full immunization a reality at the global level, the regional level but especially at the country and community level?"

Fran Kritz is a health policy reporter based in Washington, D.C., and a regular contributor to NPR. She also reports for the Washington Post and Verywell Health. Find her on X (Twitter): @fkritz