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

Author: Aimee Cunningham / Source: Science News

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…

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