Of every 1,000 people the measles virus infects, it may kill as few as one to three. In a way, this can seem merciful. But the mathematics of measles is also unforgiving. The virus is estimated to infect roughly 90 percent of the unimmunized people it encounters; each infected person may pass the infection on to as many as 12 to 18 others. In large part owing to an ongoing outbreak in South Carolina, the United States is watching those risks unfold in real time. As of last Thursday, the CDC is reporting 982 cases of measles. That count is expected to break 1,000 this week; a tracker run by researchers at Johns Hopkins University that many experts consider more reliable has ticked past 1,000 already. By the numbers alone, another death seems inevitable, and inevitable soon.
Probabilities aren’t guarantees, of course. So far, 2026 may be seeing some improvements over 2025, when the U.S. documented more than 2,200 measles cases—more than in any year since 1991. This year, just 4 percent of measles cases have led to hospitalization, compared with 11 percent last year. Several factors could be contributing to that discrepancy, including how hospitals in South Carolina are reporting measles admissions or of more mild cases being diagnosed to begin with; experts aren’t yet sure.
That 4 percent, however, still represents 40 or so people who have ended up in the hospital with at least one of the conditions that can make measles so devastating—among them, pneumonia, respiratory failure, and brain disease. In South Carolina, multiple people, including children, have been hospitalized with a form of brain swelling called encephalitis, which can lead to permanent intellectual disability or deafness, and in some cases turn fatal.
Outbreaks are brewing elsewhere in the country too—Florida, Utah, Arizona. The nation is on the verge of losing the measles-elimination status it has held for 26 years, which would officially mean that the virus was once again routinely circulating in the United States. The majority of measles cases will remain somewhat mild. But as outbreaks continue, Americans will see where percentages mislead. Even if the rates of death and disabling disease remain roughly the same, as case numbers grow, so too will the absolute amount of suffering.
The calculus of the measles vaccine, meanwhile, should be comforting: A single dose of measles-mumps-rubella (MMR) vaccine can protect people against measles for decades at rates of 93 percent; two doses can protect at 97 percent. Some vaccines work mostly to keep people from getting very sick, but the measles one is powerful enough to prevent many infections from taking hold at all. Only 150 or so of 2025’s measles cases—7 percent—occurred in people known to have received at least one MMR dose. (The CDC and Johns Hopkins haven’t been reporting on hospitalizations by vaccination status.)
If those numbers still sound uncomfortably high, consider that 90 percent of American kids have gotten at least one MMR dose. The higher the vaccine coverage, the more cases will occur among the vaccinated—but also, the far fewer cases will occur overall. And studies have consistently found that when vaccinated people do contract measles, their cases are much milder and potentially less contagious than unvaccinated cases.
Still, certain factors, including genetics and immunocompromising conditions, can alter the level of protection a person gets from an immunization. Age, too, naturally erodes defenses, especially for people decades out from their most recent measles-vaccine dose. And not all vaccinated people are vaccinated in an optimal way. Some Americans, for instance, are too old to have been vaccinated with both modern MMR doses; children generally don’t receive their second injection until they’re about to begin kindergarten. The more a virus transmits broadly, the more easily it can exploit any vulnerability it finds. During a measles outbreak that began in the Netherlands in 1999, more cases were detected in vaccinated people living in mostly unvaccinated communities than in unvaccinated people in highly vaccinated communities—simply because low-vaccine communities were giving the virus far more chances to spread.
Unvaccinated people living among other unvaccinated people remain at the highest risk, Maia Majumder, an infectious-disease modeler at Harvard Medical School and Boston Children’s Hospital, told me. The current statistics reflect that: The large majority of measles infections—93 to 94 percent—are still happening in unvaccinated people. Last year’s largest outbreak, centered on West Texas, killed two school-age children, both of whom were unvaccinated.
Other consequences of measles can take years to become obvious. Because of a quirk in its biology, the virus can erase a person’s preexisting immunity against other pathogens, leaving them more vulnerable to all sorts of illnesses. The more severe the measles infection, the more thorough the damage. Another of measles’ worst and most insidious outcomes is subacute sclerosing panencephalitis (SSPE), an untreatable neurodegenerative condition that can take nearly a decade to manifest. Alex Cvijanovich, a pediatrician in New Mexico, told me that about two decades ago, she treated a middle schooler who had caught the virus as a seven-month-old, still too young to be vaccinated. The initial illness was tame, seemingly inconsequential. But around the age of 12, the boy—an honor student—“started getting lost between his classes,” Cvijanovich said. A spinal tap eventually showed that the virus had lingered in his neural tissue for more than a decade, causing irreversible brain damage. In the following months, the boy’s nervous system deteriorated until he could no longer control the flow of fluid into his lungs. He asphyxiated on his own body’s secretions just a few years after measles had been declared eliminated in the United States.
“It was the most horrible, devastating death of all my years of training and doing pediatrics,” Cvijanovich said. “I comforted myself by telling myself, I’ll probably never see this again.”
Now she is no longer so sure. SSPE, like many other measles complications, is rare, occurring in perhaps one out of every few thousand infections among the unimmunized. (Cases among the vaccinated are virtually nonexistent.) But children who catch the virus in infancy seem especially vulnerable.
To protect their patients from infection, Cvijanovich and her colleagues keep a “rash phone” outside of their office, for families bringing in children who look especially blotchy and red, so that a nurse can inspect them far away from other vulnerable kids. James Lewis, the health officer for Snohomish County, Washington, which has been battling a smaller measles outbreak for several weeks, told me that his department has been advising any patients with suspicious symptoms and a potential measles exposure to call ahead, so they can wait outside the doctor’s office until they can be seen inside. Some may even be evaluated in their car.
Not every place has the resources for such investments, or for the testing, contact tracing, isolation rooms, vaccine clinics, and other measures necessary to help stop measles outbreaks. And some experts worry that as measles continues to appear in confined environments—such as, recently, an ICE facility in Texas—adequate infection-prevention measures will too frequently fall short.
Measles is one of the most contagious viruses ever documented and requires near-comprehensive levels of vaccination—roughly 95 percent or more—in a community to prevent it from spreading. But uptake of the MMR vaccine has ticked steadily down in recent years. Experts anticipate further drops under the Trump administration, especially as Robert F. Kennedy Jr., the secretary of the Department and Health and Human Services and a longtime anti-vaccine activist, continues to restrict access to vaccines, dismiss vaccine experts, challenge vaccine manufacturers, and question vaccine safety. (HHS did not respond to a request for comment.) One recent modeling study found that a drop in nationwide MMR uptake of just a few more percentage points could lead to millions more measles cases over the next 25 years. And the more measles moves around, the more the risk to everyone will increase.