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.

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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.

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. (Babies may 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.) 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.

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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 recorded 667 measles cases, a record since the disease's official 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 proportion of people getting 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 certain U.S. communities where many people are unvaccinated, said Nakia Clemmons, a CDC epidemiologist who conducted the analysis.

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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.

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 largest number of cases were in California (380), New York (250), Ohio (396) and Washington 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.

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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.

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.

In studying the data, researchers also looked for clues about the vaccine's effectiveness. If it were not working well, researchers would expect “to see more vaccinated cases having measles,” Clemmons said. But that was not the case.

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