The current U.S. measles outbreak follows, in some ways, a classic pattern: The virus first found a foothold where childhood vaccination is low—among Mennonites in Texas, in this case—before rapidly spreading to other communities and states. It has sickened mostly children and has now killed a second child, whose death was reported this weekend. With cases still ticking up, experts expect the outbreak to persist for a year.
Look closely at the outbreak’s edges, though, and the patterns are more unusual: It’s not just children getting measles. Where Texas’s outbreak has spilled over into New Mexico, for example, half of the confirmed cases and one potential death are in adults, largely unvaccinated. Last year, too, adults older than 20 accounted for more than a quarter of U.S. measles cases. This is all in keeping with what experts have warned: Adults are now susceptible to this childhood disease.
Doctors tend to be unfamiliar with adult measles, because adults used to not get it. In the prevaccine era, the extremely contagious virus blazed through with such frequency that virtually all children were infected with measles before reaching adulthood. Today, vaccine coverage is widespread enough that unvaccinated children can easily live to adulthood without ever encountering the virus, but not uniformly high enough to prevent outbreaks altogether. Vaccinated adults can get occasional breakthrough cases, but the illness tends to be much milder. Unvaccinated adults, however, are a uniquely vulnerable group, because measles only becomes nastier and deadlier with age.
The risk from measles follows a U-shaped curve, Neal Halsey, a measles expert and professor emeritus at Johns Hopkins, told me. On the left are children under 5, whose still-maturing immune systems might struggle to fend off the virus. On the right are adults; the rise in mortality gets steeper and steeper over time, eventually surpassing the mortality in young children. When measles managed to reach isolated villages in the past—such as when a sick sailor brought it to a remote part of Greenland in 1951—outbreaks on “virgin soil” proved especially deadly for adults. Of the 77 people who died in Greenland then, 59 were over the age of 35. Measles may be a classic childhood illness, but it mostly killed adults there.
This age gradient of measles severity persists today. Although the typical symptoms of rash, fever, and cough are the same, adults—even healthy adults in their 20s and 30s—grow more prone to the severe complications that might prove dangerous and even fatal. Pneumonia and encephalitis, or infections of the lungs and brain respectively, are more common. One in four adults with measles will need to be hospitalized, a rate that is roughly two to three times that of school-age children.
Matthew Goetz still has “vivid memories” of adult patients he treated as an infectious-disease doctor during a 1988–90 outbreak in Los Angeles. The first patient wasn’t diagnosed until after a couple of days in the hospital, he recalls, because doctors had little reason to suspect that an adult would have measles. Several more soon showed up. Of the 33 patients eventually admitted to the public hospital where Goetz worked, nine had to be transferred to the ICU. Six developed respiratory failure so severe—presumably from pneumonia—that they needed a ventilator.
Why measles gets deadlier with age of first contact is still not entirely understood; the adult immune system must somehow be less optimized to fight off the virus. Halsey points out that this pattern is not unique to measles: Chickenpox and hepatitis A are also milder in children than in adults. So is COVID, as we’ve recently seen.
The consequences of measles can linger long after infection, too. Measles has a singular ability to induce “immune amnesia,” making survivors potentially susceptible to other diseases they’ve already had or been vaccinated against. This is because the virus attacks immune cells, including memory B cells, which “remember” how to fight known pathogens through antibodies. A 2019 study found that a course of measles infection in unvaccinated children wrecked 11 to 73 percent of their antibody repertoire. This range suggests that immune amnesia’s impact may vary widely from person to person, but the overall trend explains some old and odd observations about postmeasles immune suppression. For example, measles can make autoimmune diseases, in which the immune system mistakenly attacks one’s own body, go into remission. Immune amnesia also explains, at least in part, a long-standing pattern of children becoming more vulnerable to other illnesses after getting measles.
This effect has largely been studied in children, so scientists do not really know how it affects adults. “I would anticipate that it would be very similar—and it also might even be slightly worse,” says Stephen Elledge, a biologist at Harvard and senior author on the 2019 immune-amnesia study. A course of measles tends to last longer and be more severe in adults, he reasons, so the disease may kill off more of their memory cells. He suggests that anyone who gets measles should get revaccinated for other diseases, just in case.
If the measles vaccination rate dips further, adult cases could become even more common. The U.S. eliminated measles in 2000, after many years of achieving a better than 95 percent vaccination rate among kindergartners. This number started slipping in 2020 and is now down to 92.7 percent, which is, importantly, edging toward the measles herd-immunity threshold of 92 to 94 percent. Under this threshold, herd immunity can no longer limit spread enough to protect the unvaccinated. A bigger pool of unvaccinated kindergartners means a greater potential for outbreaks that grow massive enough to threaten unvaccinated adults. And if those unvaccinated kids never get their shots later in life, they will become susceptible adults, growing more vulnerable to measles with age.
For now, the Texas outbreak is already so widespread that the U.S. is likely to lose its measles-elimination status. In the time it will take to get this outbreak under control, more children will certainly get infected, as will more adults.
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