Despite being declared eliminated in the U.S. a quarter century ago, measles has infected 146 people in northwest Texas in the past two months, killing one child. Other smaller outbreaks are happening in New Mexico, California, Georgia, New Jersey, Rhode Island, and other states. Most of the cases are in children, but more than two dozen of the Texas patients are adults. What most infected individuals have in common is that the vast majority were not protected by the measles vaccine.
Vaccination is the only effective prevention for measles, a viral respiratory disease that spreads quickly, causes severe illness, and has no known cure. Since 1971, measles immunizations are given in a combination shot with mumps and rubella. This MMR vaccine makes it easier for doctors to store the vaccines and for patients to receive them. Yet vaccination rates have been declining in the U.S. in recent years, giving measles opportunities to resurge.
Infectious disease experts suggest everyone should know their vaccination status—a prospect that can be especially challenging for some older adults who got the shots when they first became available but may have received a version later deemed ineffective.
“There certainly could be people thinking they’re immune and they may not be,” says Adam Ratner, a pediatric infectious diseases doctor at the Hassenfeld Children’s Hospital at NYU Langone in New York and author of the book Booster Shots, on the history of measles.
What makes measles so dangerous?
Measles is among the most contagious diseases on the planet. An infected person spreads the virus to an average of 12 and 18 people. If they enter a room with 100 others who are unvaccinated, 90 of them develop the disease. The airborne virus can linger up to two hours, so the infected person can even sicken others long after they go elsewhere.
Another difference between measles and other respiratory ailments: nearly everyone who contracts measles develops full-blown symptoms, which generally appear one to two weeks after exposure. “There’s not a version of the disease where you might have a few sniffles,” says Lori Handy, associate director of the Vaccine Education Center at the Children’s Hospital of Philadelphia.
Telltale rashes on the face and body are most associated with the condition, but other symptoms include cough, runny nose, fever (sometimes higher than 104° F), and painful eye redness. “Even people who don’t need to be hospitalized are pretty miserable,” Ratner says.
Some patients develop more severe complications, including pneumonia or brain swelling, known as encephalitis. So far this year, 25 percent of people with the disease have required hospitalization, according to the U.S. Centers for Disease Control and Prevention. Some end up with permanent hearing loss or blindness. Measles can also alter a person’s immune system’s B-cells so they forget how to fight other diseases they previously encountered. This dangerous “immune amnesia” can last for years.
(Measles vaccines protect against more than just measles.)
Measles can also be fatal, with a one-in-a-thousand death rate—compared, for example, to flu’s two in 100,000. The death of the Texas child, who was unvaccinated, marks the first in the U.S. since 2015. Before vaccines, some 400 to 500 people in the U.S. died from the condition each year.
Who is most at risk for measles?
Children are most susceptible. That’s why experts recommend that all kids receive a shot at around one year of age and a second before kindergarten, although catch-up vaccines are available at any age.
In the Texas outbreak, 74 percent of patients are younger than 18. While differences in their immune systems may play some role, the prevalence likely reflects reductions in childhood vaccination rates as some parents question their safety. Some 8 percent of eligible children nationwide were un- or under-vaccinated for measles as of 2019, the most recent figures; in some communities, including in Texas, the number is significantly higher. The COVID pandemic may have also delayed normal shot schedules.
(How the anti-vaccine movement got its start.)
The higher disease burden in kids also reflects the amount of time they spend together, says Priya Sampathkumar, an infectious disease specialist at the Mayo Clinic in Rochester, Minnesota. “Children in daycare and schools are in very close contact with one another. And because the second dose isn’t given until four or five years old, younger children are less protected,” she says.
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Other populations at increased risk include people raised in countries that did not routinely immunize against the disease and the three or so percent who, when given proper vaccines, fail to mount an effective immune response. This can happen even with a healthy immune system, so someone may not realize there’s a problem, Ratner says.
For older people, the specific type of measles vaccine they received in childhood could also affect their current immunity.
What if you got one of the early measles vaccines?
The U.S. Food and Drug Administration approved the first measles vaccines in 1963, and two shots were introduced that year. Merck produced a live vaccine, meaning a small amount of weakened active virus remains after processing—enough to trigger an immune response without conveying the actual disease. Pfizer’s version was an inactivated shot, altered in a way that no living virus is present.
Some 15 million American kids got one of the two versions in the first few years. By 1966, a disease that had previously sickened half a million people a year had already fallen by half.
In 1967, however, researchers realized that immunity from the inactivated shots dropped dramatically in as little as six months, while the live vaccine protected people for years. The Pfizer shot was soon removed from the market—which might come as a surprise to some who were vaccinated as children during this period.
That’s why the CDC currently advises that anyone immunized before 1968 and unsure of the version they received get at least one updated shot. People approximately 59 to 68 years old would be eligible for this “booster,” which is a regular dose of the MMR vaccine. Others who received one measles shot before the two-dose regimen was introduced in 1989 have as much as 93 percent protection, but during major outbreaks public health officials may recommend getting a second dose. People born before 1957 do not need a vaccine; they likely have immunity from childhood exposure to the then-still-prevalent disease.
While age is one way to calculate your vaccine needs, another option is to ask your physician for a blood test that indicates whether you’re protected. “It’s not 100 percent accurate, but it’s pretty good,” Ratner says.
Studies in recent years confirm that immunity to the two-dose live vaccine lasts a lifetime. An observational study in France calculated that effectiveness diminishes just 0.22 percent each year. A modeling study in the United Kingdom of teens and young adults put the rate at approximately 0.04 percent yearly. At that rate, 45 years after vaccination someone would have still have 98 percent of their original protection.
“If immunity waned quickly, we’d be seeing much bigger outbreaks now,” says Alexis Robert, a mathematical modeler in infectious diseases at the London School of Hygiene & Tropical Medicine and the British study’s lead author. Older people would also make up large numbers of current cases, which is not occurring.
Vaccination is a way to protect others
But not everyone can get the shot. Because MMR is a live vaccine, people with weakened immune systems from cancer therapy or certain diseases run the risk of falling ill. Neither can pregnant women or those with allergies to vaccine ingredients. Widespread measles immunization is an important way to protect people who can’t be vaccinated, since the virus struggles to spread when enough people have immunity. For measles, the vaccination threshold for this firewall is around 95 percent in a community.
In addition to preventing measles, the vaccine may make the rare person who gets the two-dose shot but still contracts the disease less contagious. This happened in Spain in 2017, when an emergency room nurse unknowingly infected in the hospital flew on a plane and lived with others before becoming symptomatic, yet he didn’t pass the disease to anyone.
More people in the U.S. have become hesitant to vaccinate themselves or their children in recent decades. Dropping vaccination rates have driven past U.S. outbreaks, and in 2019, threatened the country’s elimination status, a designation that means when cases happen, they’re stamped out within a year.
Some cite the risks of the MMR vaccine, which are mild and include arm soreness, fatigue, fever or a slight rash. A small number find their blood platelet level drops, but this resolves quickly. Others point to long-disproven claims based on fabricated data that the vaccine is linked to autism.
Side effects for every disease must always be weighed against the dangers of contracting the condition, Handy says. “Now that we have circulating measles—with the pneumonia, encephalitis, hospitalization, and death,” she says, “folks should absolutely opt for the very safe, very effective choice of the vaccine.”
