Measles elimination -- absence of year-round transmission -- was declared in 2000 in the United States, but measles is imported into the country from countries where it is still common.
Eight outbreaks and 159 cases of measles were reported to the U.S. Centers for Disease Control between Jan. 1 and Aug. 24. An outbreak in New York City was the largest U.S. outbreak since 1996 at 58 cases, the CDC said.
Ninety-nine percent of the measles cases were from people from other countries or from Americans who visited other countries, while 82 percent were unvaccinated and 9 percent had unknown vaccination status, the report said.
Among U.S. residents who were unvaccinated, 79 percent said they had philosophical objection to vaccination.
High vaccine coverage is important to prevent spread of measles following importation, researchers said.
The findings were published in the CDC's Morbidity and Mortality Weekly Report.