A steady uptick in the number of measles cases in the U.S. in recent years has primarily been driven by people who haven’t been vaccinated.
Those are the findings of a new report published in JAMA Pediatrics which found that measles, which was eradicated nationally in 2000, has made a return largely because of parents who have refused to vaccinate their children.
In the latest findings, researchers from the Centers for Disease Control and Prevention analyzed 1,789 measles cases among U.S. residents reported to the CDC from January 2001 through December 2015. They found that nearly 70 percent, or 1,243 individuals, were unvaccinated.
Babies and toddlers had the least protection. Of 163 infants ages 6 to 11 months who became sick, only two had been vaccinated. Among 106 toddlers ages 12 to 15 months, 95 were unvaccinated.
The study found that the rate of measles has accelerated with time, with 667 cases recorded in 2014, a record since it was declared eradicated. The authors’ works shows another troubling trend: the proportion of cases caused by unvaccinated travelers from overseas has declined over time, with just 15 percent of cases imported to the country in 2015 compared to 47 percent in 2001.
The trend is significant because it may suggest “increased susceptibility and transmission” in certain U.S. communities where many people are unvaccinated, said Nakia Clemmons, a CDC epidemiologist who conducted the analysis.
A common scenario is this: A family leaves the country on vacation and one child gets infected and develops measles upon returning to the United States. “Then the child goes to a play group with other kids who are unvaccinated, and those kids catch measles,” said Saad Omer, a professor of global health, epidemiology and pediatrics at Emory University.
The CDC recommends that children get two doses of the Measles, Mumps and Rubella vaccine (MMR), with the first dose coming between 12 and 15 months of age, and the second between the ages of 4 and 6. Those who are not vaccinated risk falling ill if exposed to the disease, with symptoms generally appearing about 7 to 14 days after a person is infected. The illness typically begins with high fever, cough, runny nose, and red, watery eyes. For most people, symptoms last around 7 to 10 days.
During the study period, the largest number of cases reported were in California with 380, New York with 250, Ohio with 396 and Washington state with 102. Some communities in those states were hit particularly hard, such as the outbreak among unvaccinated Amish residents in Ohio in 2014.
Although measles vaccination rates remain high overall nationally, there are communities across the country where vaccine coverage is slipping below the 90 percent to 95 percent level that experts say is needed to prevent an outbreak.
While the authors said one limitation of their study was the inability to verify immunization on nearly half of the adult cases reviewed during the analysis, public health officials like Omar said the findings still underscore the urgent need to vaccinate against preventable diseases.
“Americans should get vaccinated and make sure that we maintain this social norm in our play groups, in all of our communities,” Omer said.
The spread of measles in communities of unvaccinated people increases the risk for transmission to vulnerable groups, such as people who can’t be vaccinated because of underlying medical conditions or babies too young to be vaccinated.