How holes in herd immunity led to a 25-year high in U.S. measles cases

Measles is so contagious that the virus can unerringly find the unvaccinated. That knack, combined with the number of people in the United States who haven’t been vaccinated against the disease, has given measles an opening that it hasn’t had in the country for decades.

U.S. measles cases have surged to a 25-year high of 704, the U.S. Centers for Disease Control and Prevention reported April 29. The record-breaking number of cases largely stems from an outbreak in Washington, which officials now say is over, and ongoing outbreaks in New York City and New York state. The vast majority of the cases, 88 percent, originated in close-knit communities that have low rates of vaccination, according to the CDC. And 503 of the 704 cases were in those who have not been vaccinated.

The virus is eliminated in the United States, which means that it is no longer endemic, but travelers can still bring it into the country. “When measles is imported into a community with a highly vaccinated population, outbreaks either don’t happen or are small,” Nancy Messonnier, the director of CDC’s National Center for Immunization and Respiratory Diseases, said in a news briefing.

But once introduced into a community with low vaccine coverage, “it is difficult to control the spread of disease,” she said.

Cases are reported in 22 states, with new outbreaks (defined as at least three cases) in Maryland, Georgia and Los Angeles County. The six infected people in that county included students who potentially exposed the campuses of UCLA and California State University, Los Angeles to measles. To contain the outbreak, health officials told hundreds of students and staff to stay home until they could prove that they’d been vaccinated.

The vulnerable pockets of people who aren’t vaccinated against measles tend to have a common denominator, sharing similar values, religion or culture. And the age of the students at the two college campuses gives a clue to what binds them: Some were born around 1998, the year a now-debunked study came out questioning the safety of the measles-mumps-rubella vaccine (SN Online: 2/3/10), possibly dissuading some of their parents from vaccinating their kids.

Also hit are Orthodox Jewish communities, in outbreaks that began in 2018 in New York, while the Washington outbreak heavily affected Russian and Ukrainian communities. In recent years, measles has also spread among Somali-Americans in Minnesota, the Amish in Ohio and visitors to Disneyland in California.

The people in the exposed communities either reside close together or gather to socialize, says Saad Omer, a vaccine researcher at Emory University in Atlanta. But the cultural or religious characteristic that brings them together isn’t why some aren’t vaccinated, he says, but rather underlies why they are in close contact. “We need to make sure that communities … do not get stigmatized,” he says, as they “are victims of both misinformation and disease.”

In fact, most of the people in these communities are getting vaccinated, Omer says. “It’s just that that number may not be high enough for an unforgiving disease like measles … it spreads so easily that even a modest drop in coverage” can set the stage for an outbreak.

To stop the spread of measles, around 92 to 95 percent of a population needs to be vaccinated to preserve its herd immunity, meaning that there are enough vaccinated people that the pathogen runs out of new people to infect and transmission stops. For the 2017–2018 school year, state-by-state coverage for the two recommended shots of the measles-mumps-rubella vaccine among kindergartners ranged from a high of 99 percent in Mississippi to a low of 81 percent in Washington, D.C., according to the CDC.

Using data from 2008 to 2013 from the CDC’s national survey of teen vaccination history, Omer and his colleagues have tallied how many measles-susceptible children there are in the United States. Those numbers include those who can’t get shots because of treatments that suppress their immune systems, those too young to get vaccinated and those who haven’t received their shots by kindergarten.

Of nearly 70 million children age 17 and younger during that time period, 12.5 percent — or an estimated 8.7 million — were not protected against measles, Omer and colleagues reported in 2016 in the American Journal of Epidemiology. Fourteen states, including Washington, Michigan, Texas, California, Arizona and New York, had 20,000 or more children ages 13 to 17 unvaccinated for measles as of 2013.

“You’re only as safe as your herd is immune,” says Yvonne Maldonado, a pediatric infectious disease expert at Stanford University School of Medicine. “If you have a low immunity herd … the likelihood of being exposed is going to be much higher.”

For kids who can’t be vaccinated, getting the measles “can be fatal or wind up causing serious complications,” she says. “That’s a real fear for [these] families.” Herd immunity provides the only protection for these children.

The outbreaks are an opportunity to renew vaccination efforts, she says. That could ensure that measles doesn’t once again become endemic in the United States. “Most people really do want to protect their kids against these diseases.”