As the nation’s measles cases approach 3300, at least one aspect of the continuing outbreaks has become clear: the spread of the virus and the consequences of infection are not limited just to children.

Thirty-seven percent of the cases confirmed by the CDC in 2026 have been in individuals older than 20 years. As of September 11, more adults (classified as individuals older than 20 years) than children had been hospitalized — 144 compared with 123. The agency acknowledges that its figures lag what is being reported by states.

In Pennsylvania, for instance, which has an ever-growing outbreak, 93 people older than 18 years have been hospitalized with measles, accounting for 20% of the 474 cases in that age group. Just 15% of those younger than 18 years (31 of 202 cases) have been hospitalized.

Two children in the state have reportedly died from measles. On September 12, the Jefferson County Pennsylvania Coroner’s Office reported that a 40-year-old woman died “from complications associated with measles.” The office did not say whether she had received any measles, mumps, rubella (MMR) vaccine doses.

The accumulating data points to the need for a mindset shift, Gregory Poland, MD, president and co-director of the New York-based Atria Research and Global Health Institute, told Medscape Medical News.

“Measles is not just a pediatric disease,” said Poland, a vaccine researcher.

The nation may soon start seeing more cases in adults, said Robert Bednarczyk, PhD, associate professor of Global Health and Epidemiology at the Emory University Rollins School of Public Health in Atlanta.

The growing number of cases in older children and young adults is a sign of vaccine hesitation coming home to roost, both Poland and Bednarczyk told Medscape Medical News. Not only has under vaccination “been going on for a long time,” but unvaccinated children are now growing into unvaccinated adults, Bednarczyk said.

Bednarczyk and an Emory colleague estimate that declining vaccination rates — likely due in part to vaccine hesitation — would increase the proportion of children and adolescents susceptible to measles from 13.5% to 21.4%.

William Schaffner, MD, a spokesman for the Infectious Diseases Society of America, was less certain that measles would take off widely in adults. With past more-recent outbreaks, “there’s been very little spread into the general vaccinated community,” because of the durability of the vaccine’s protection and the fact that people born before 1957 are considered immune due to exposure during childhood, Schaffner told Medscape Medical News.

“But it could very well be a localized problem for adults and young adults who have been brought up in under-vaccinated communities,” said Schaffner, professor of infectious diseases at Vanderbilt University School of Medicine in Nashville, Tennessee.

Jason Goldman, MD, an internist in Coral Springs, Florida, and immediate past president of the American College of Physicians, agreed that communities with lower vaccination rates would be more vulnerable.

But that doesn’t rule out spread to adults in other areas “because it is such a pervasive virus,” Goldman told Medscape Medical News, noting that it can live on surfaces for extended periods and that it can be a week or more before a person is symptomatic.

Pool of Adults at Risk

Two doses of MMR vaccine are 97% protective against measles, and those who are vaccinated will have a milder form of the disease if they are infected, said Schaffner.

But many adults are considered susceptible to infection and the deleterious effects of the virus. From 1963 to 1967, many American children received a less-effective vaccine that had an inactivated virus. Immunity may not be as strong in those adults or may have waned. Prior to 1989, a single dose of the vaccine was standard practice. One dose confers up to 93% protection, but again, it leaves some room for infection.

And that’s a concern as cases spread.

Measles “can be a wicked disease,” said Poland. “We have no treatment,” he added, noting that there are no measles antivirals or monoclonal antibody therapies because there was no market — the disease had been eliminated in the US. “We are at roughly the same ability to ‘treat’ measles as we were when I was born in the 50s,” said Poland.

What’s Known About Complications in Adults

It has long been known that “the youngest kids and adults 20 and older are at the highest risk for complications but for different reasons,” said Bednarczyk.

Children younger than 5 years either have not been vaccinated or have vulnerable immune systems, he said. In adults — especially if unvaccinated — infection can overwhelm the immune system, leading to catastrophic pneumonitis and encephalitis, he said.

In a French study of 460 adults hospitalized during a 2010-2011 outbreak, a third had pneumonia and two thirds had elevated serum transaminases. One patient died from pneumonia. Another paper on that French outbreak examined 36 patients admitted to the ICU. Half were admitted because of respiratory complications; 11 required mechanical ventilation. Among 26 who had documented vaccination status, none had received two injections.

Among 93, primarily young and healthy patients with confirmed measles hospitalized in Greece from 2017 to 2019, pneumonitis (46.2%) and hepatic involvement (80.6%) were the most common complications. Twelve reported no vaccination, 56 had an unknown status, and 25 had only one MMR dose. One of four pregnant woman had spontaneous fetal miscarriage, and an additional patient died due to acute respiratory distress syndrome and high-risk pulmonary embolism.

In a 2023-2024 outbreak in Romania, researchers reported that liver damage was more common in adults than in children (63% vs 34%). Moderate-to-severe cytolysis was predominantly observed in adults (34.9% vs 1.9%; P < .001), and thrombocytopenia was significantly more frequent in adults (P < .001).

MMR Vaccine Is Still Key

Vaccination is still the best protection, which means clinicians should be asking patients about immune status whenever possible.

Even if adults can’t remember or document whether they received a vaccine in childhood, Schaffner said they should get another MMR dose. “That’s the easiest thing,” he said.

Poland said asking about immunization could be difficult if patients are vaccine hesitant. It means building trust with patients over time, talking about the data, and understanding their cognitive biases — an empathic process he and his daughter described in a paper published during the COVID pandemic.

Another issue: pregnant women, those living with chronic immune-suppressing infection like HIV, or who take immune-suppressing medications, or who have cancer, cannot receive boosters because the MMR is a live virus vaccine.

Universal vaccination is necessary to protect these vulnerable individuals, said Goldman. “We need to protect not only ourselves, but there’s a certain social contract that we have to protect our fellow human beings,” he said.

As cases rise, “We’re going to find out how many people are at risk,” he said.

“It is heartbreaking to see how far we’ve fallen,” said Goldman.

Bednarczyk, Schaffner, Poland, and Goldman reported having no disclosures.

Alicia Ault is a Saint Petersburg, Florida-based freelance journalist whose work has appeared in many health and science publications, including Smithsonian.com. You can find her on X @aliciaault and on Bluesky @aliciaault.bsky.social.