Measles, a disease preventable by vaccination, is resurgent in parts of Europe, including Britain. Small outbreaks have also popped up in multiple parts of the United States.

In Europe, reported measles cases rose more than 40-fold last year compared with 2022, the World Health Organization said on Tuesday. Nearly a third of those cases were in Kazakhstan, where the outbreak is attributed largely to children who missed routine immunizations. (The country straddles both Europe and Asia; the W.H.O. considers it part of the “European Region.”) Experts fear the virus could spread beyond Kazakhstan.

Most cases in the United States have been linked to travel outside the country. The number of cases reported last year was lower than in most years before the pandemic.

A close view of a single vial of measles, mumps and rubella vaccine resting on an orange fact sheet about the vaccine.
A single dose of the measles vaccine is 93 percent effective at stopping the virus, according to the Centers for Disease Control and Prevention.Credit…Brian Snyder/Reuters

Some cases of measles can be mild, but up to half of infected children may need medical attention, said Dr. David Sugerman, who leads the measles team at the Centers for Disease Control and Prevention.

Children with measles may develop diarrhea and dehydration, pneumonia that leads to long-term respiratory difficulties, and brain inflammation that results in neurological problems, Dr. Sugerman said.

For every 1,000 cases in children, one child may become deaf or intellectually disabled, and one to three may die. Deaths from measles rose worldwide by 43 percent between 2021 and 2022, according to a report in November from the W.H.O. and the C.D.C.

Measles is among the most contagious infections, and the virus can linger in the air for up to two hours. “It is so contagious that if one person has it, up to 90 percent of the people close to him or her will also become infected if they are not protected,” Dr. Sugerman said.

The disease is characterized by respiratory symptoms, fever, conjunctivitis and a rash that can be mistaken for roseola, scarlet fever or other viral infections.

In the United States, the measles vaccine is given twice, at 12 to 15 months old, and at 4 to 6 years of age. Even a single dose is 93 percent effective at preventing the disease, according to the C.D.C.

Dr. Sugerman urged families planning to travel, or who are otherwise worried about exposure, to immunize infants at 6 to 11 months old, and to reconsider those plans if the infants are younger than 6 months.

A false claim in the 1990s that said the combined measles, mumps and rubella vaccine causes autism led to a drop in immunization rates. Public health campaigns later recouped much of that deficit, but the rates again fell during the Covid-19 pandemic, particularly in low-income countries.

The measles virus is particularly adept at finding pockets of vulnerability, but outbreaks of other vaccine-preventable diseases may follow, said Dr. Saad Omer, the dean of the O’Donnell School of Public Health at U.T. Southwestern in Dallas.

“Measles is usually the canary in the coal mine,” Dr. Omer said.

In the United States this year, Philadelphia has recorded nine cases of measles, Washington State confirmed three cases and was investigating three others, and several states were tracing contacts of a single case each.

As of January, 49 countries have what the W.H.O. calls “large or disruptive outbreaks,” according to Dr. Natasha Crowcroft, senior adviser on measles and rubella at the organization.

Britain confirmed 250 measles cases in 2023, most of them in children younger than 10. In Europe last year, one in five cases was an adult 20 years or older, according to the W.H.O.

Those numbers may seem modest, but they are a sign that public health officials should step up immunization campaigns, Dr. Omer said.

“If a fire is just starting, that’s the time to go all in and douse it right away, rather than wait for it to spread,” he said.

For measles to remain under control, at least 95 percent of a population must be immunized. In Europe, the percentage of people who had received a first dose dropped from 96 percent in 2019 to 93 percent in 2022.

More than 1.8 million infants missed their measles vaccinations between 2020 and 2022.

“We actually knew this was going to happen, so it’s not news for us,” Dr. Crowcroft said of Europe’s rise in measles cases.

“There are times when there’s absolutely no pleasure in being right, and this is one of those.”

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