Failure to vaccinate linked tomeasles outbreaks

Findings of analysis showmost cases brought into U.S.

Peoplewhodon’t get vaccinated are the most likely reason for the steady increase in the rate of measles and majoroutbreaksintheUnited 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 spreadofthehighlyinfectious and potentially fatal disease. Once commonin theUnited States, measles was eliminated nationally in 2000but 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, wheremeasles may remain endemic.

The 2014-2015 outbreak that originated atDisneyland most likely started when a travelerwhobecameinfected overseas visited the theme park.

In the latest findings, researchers from the Centers for Disease Control and Prevention analyzed 1,789 measlescasesamongU.S. residents reported to the CDC from January 2001 through December 2015. They found that nearly 70 percent, or 1,243individuals, wereunvaccinated.

Babies and toddlers had the least protection. Of 163 infants ages 6 to 11 months who became sick, only two hadbeen vaccinated. Among 106 toddlers ages 12 to 15 months, 95 were unvaccinated.

Federalguidelinestypically recommendthat childrenget their first vaccine dose at 12 to 15 months of age and the second when they are ages 4 to 6. (Babiesmay be vaccinated at 6 months or older if they are at risk of exposure to measles, by traveling to an area with an outbreak, for example.)

Twodosesofvaccineare97 percent effective in preventing the viral disease, which can cause pneumonia, brain swelling, deafness and, in rare instances, death.

The authors said the rate of measles increased over time, with 10 of 13 outbreaks with at least 20 cases occurring after 2010.

In 2014, the United States recorded667measlescases, a recordsince the disease’sofficial elimination. One major outbreak that year occurred primarily among unvaccinated Amish communities in Ohio.

The analysis also shows that the proportion of imported cases has declined over time, while the proportionofpeoplegetting infected in the United States rose. Imported cases of measles fell from almost 47 percent of all cases in 2001 to about 15 percent in 2015, the data show.

The trend is significant because it may suggest “increased susceptibility and transmission” in certainU.S. communities where many peopleareunvaccinated, said Nakia Clemmons, aCDCepidemiologistwho conducted the analysis.

Acommonscenario is this: A family leaves the country on vacation and one child gets infected and develops measlesuponreturningtothe UnitedStates.“Thenthechild goes to a play group with other kidswho are unvaccinated, and those kids catch measles,” said Saad Omer, a professor of global health, epidemiology and pediatrics at Emory University.

Omer, who was not involved in the analysis, said the data illustrate the increasing number of “pockets of vulnerability” in the United States. Cases typically occur in such communities as well as in metropolitan areas or those with major ports of entry. During the 15-year period studied, the largestnumberof caseswere in California (380), NewYork (250), Ohio (396) andWashington state (102).

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 healthofficials said the bottomline 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.

The spread of measles in communitiesofunvaccinated people increases the risk for transmission to vulnerable groups, such as peoplewho can’t be vaccinated because of underlying medical conditions or babies too young to be vaccinated.

In studying the data, researchers also looked for clues about the vaccine’s effectiveness.

If itwere notworkingwell, researcherswould expect “to see more vaccinated cases havingmeasles,” Clemmons said. But that was not the case.

And if vaccine performance had decreased over time, researcherswouldhave expected to see higher rates of measles among older age groups but that was also not the case, she said.