People who don’t get vaccinated are the most likely reason for the steady increase in the rate of measles and major outbreaks in the United States, according to an analysis released Tuesday.
The findings, published in JAMA Pediatrics, add to the body of evidence linking failure to vaccinate with the spread of the highly infectious and potentially fatal disease. Once common in the United States, measles was eliminated nationally in 2000 but has made a return in recent years largely because of people who reject vaccinating their children, experts say.
Most of those cases occur when the disease is brought into the country by unvaccinated people who get infected in other countries, where measles may remain endemic. The 2014-2015 outbreak that originated at Disneyland most likely started when a traveler who became infected overseas visited the theme park.
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.
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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.
Federal guidelines typically recommend that children get their first vaccine dose at 12 to 15 months of age and the second when they are ages 4 to 6. Two doses of vaccine are 97 percent effective in preventing the viral disease, which can cause pneumonia, brain swelling, deafness and, in rare instances, death.
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.
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.
The authors said one limitation of their analysis was the lack of verifiable immunization on nearly half of the adult cases. Still, public health officials said the bottom line is clear.
“Americans should get vaccinated and make sure that we maintain this social norm in our play groups, in all of our communities,” Omer said.