There’s a measles outbreak happening close to where I live. Since the start of this year, 34 cases have been confirmed in Enfield, a northern borough of London. Most of those affected are children under the age of 11. One in five have needed hospital treatment.

It’s another worrying development for an incredibly contagious and potentially fatal disease. Since October last year, 962 cases of measles have been confirmed in South Carolina. Large outbreaks (with more than 50 confirmed cases) are underway in four US states. Smaller outbreaks are being reported in another 12 states.

The vast majority of these cases have been children who were not fully vaccinated. Vaccine hesitancy is thought to be a significant reason children are missing out on important vaccines—the World Health Organization described it as one of the 10 leading threats to global health in 2019. And if we’re seeing more measles cases now, we might expect to soon see more cases of other vaccine-preventable infections, including some that can cause liver cancer or meningitis.

Some people will always argue that measles is not a big deal—that infections used to be common, and most people survived them and did just fine. It is true that in most cases kids do recover well from the virus. But not always.

Measles symptoms tend to start with a fever and a runny nose. The telltale rash comes later. In some cases, severe complications develop. They can include pneumonia, blindness, and inflammation of the brain. Some people won’t develop complications until years later. In rare cases, the disease can be fatal.

Before the measles vaccine was introduced, in 1963, measles epidemics occurred every two to three years, according to the WHO. Back then, around 2.6 million people died from measles every year. Since it was introduced, the measles vaccine is thought to have prevented almost 59 million deaths.

But vaccination rates have been lagging, says Anne Zink, an emergency medicine physician and clinical fellow at the Yale School of Public Health. “We’ve seen a slow decline in people who are willing to get vaccinated against measles for some time,” she says. “As we get more and more people who are at risk because they’re unvaccinated, the higher the chances that the disease can then spread and take off.”

Vaccination rates need to be at 95% to prevent measles outbreaks. But rates are well below that level in some regions. Across South Carolina, the proportion of kindergartners who received both doses of the MMR vaccine, which protects against measles as well as mumps and rubella, has dropped steadily over the last five years, from 94% in 2020-2021 to 91% in 2024-2025. Some schools in the state have coverage rates as low as 20%, state epidemiologist Linda Bell told reporters last month.

Vaccination rates are low in London, too. Fewer than 70% of children have received both doses of their MMR by the time they turn five, according to the UK Health Security Agency. In some boroughs, vaccination rates are as low as 58%. So perhaps it’s not surprising we’re seeing outbreaks.

The UK is one of six countries to have lost their measles elimination status last month, along with Spain, Austria, Armenia, Azerbaijan, and Uzbekistan. Canada lost its elimination status last year.

The highly contagious measles could be a bellwether for other vaccine-preventable diseases. Zink is already seeing signs. She points to a case of polio that paralyzed a man in New York in 2022. That happened when rates of polio vaccination were low, she says. “Polio is a great example of … a disease that is primarily asymptomatic, and most people don’t have any symptoms whatsoever, but for the people who do get symptoms, it can be life-threatening.”

Then there’s mumps—another disease the MMR vaccine protects against. It’s another one of those infections that can be symptom-free and harmless in some, especially children, but nasty for others. It can cause a painful swelling of the testes, and other complications include brain swelling and deafness. (From my personal experience of being hospitalized with mumps, I can attest that even “mild” infections are pretty horrible.)

Mumps is less contagious than measles, so we might expect a delay between an uptick in measles cases and the spread of mumps, says Zink. But she says that she’s more concerned about hepatitis B.

“It lives on surfaces for a long period of time, and if you’re not vaccinated against it and you’re exposed to it as a kid, you’re at a really high risk of developing liver cancer and death,” she says.

Zink was formerly chief medical officer of Alaska, a state that in the 1970s had the world’s highest rate of childhood liver cancer caused by hepatitis B. Screening and universal newborn vaccination programs eliminated the virus’s spread.

Public health experts worry that the current US administration’s position on vaccines may contribute to the decline in vaccine uptake. Last month the US Centers for Disease Control and Prevention approved changes to childhood vaccination recommendations. The agency no longer recommends the hepatitis B vaccine for all newborns. The chair of the CDC’s vaccine advisory panel has also questioned broad vaccine recommendations for polio.

Even vitamin injections are being refused by parents, says Zink. A shot of vitamin K at birth can help prevent severe bleeding in some babies. But recent research suggests that parents of 5% of newborns are refusing it (up from 2.9% in 2017).

“I can’t tell you how many of my pediatric [doctor] friends have told me about having to care for a kiddo in the ICU with … bleeding into their brain because the kid didn’t get vitamin K at birth,” says Zink. “And that can kill kids, [or have] lifelong, devastating, stroke-like symptoms.”

All this paints a pretty bleak picture for children’s health. But things can change. Vaccination can still offer protection to plenty of people at risk of infection. South Carolina’s Department of Public Health is offering free MMR vaccinations to residents at mobile clinics.

“It’s easy to think ‘It’s not going to be me,’” says Zink. “Seeing kiddos who don’t have the agency to make decisions [about vaccination] being so sick from vaccine-preventable diseases, to me, is one of the most challenging things of practicing medicine.”

This article first appeared in The Checkup, MIT Technology Review’s weekly biotech newsletter. To receive it in your inbox every Thursday, and read articles like this first, sign up here.