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Despite progress toward eliminating measles in the United States, outbreaks continue to occur, particularly in communities with high percentages of people who are not vaccinated because of religious and philosophical beliefs, according to a new report from the Centers for Disease Control and Prevention (CDC).
Since the beginning of this year, 159 cases of measles have been reported in 16 states.
Eight outbreaks of measles have occurred this year, including one with 58 cases in New York City, the largest U.S. outbreak reported since 1996.
The outbreaks often begin when the virus is brought into the country by travelers. So far this year, 42 people contracted measles while abroad, about half of whom had traveled to Europe, according to the report.
Three of this year's outbreaks occurred after measles was brought into communities with high rates of people who were not vaccinated due to philosophical or religious objection. These three outbreaks have accounted for nearly two-thirds of all 2013 U.S. measles cases. [5 Dangerous Vaccination Myths]
"The increase in measles cases in the United States in 2013 serves as a reminder that imported measles cases can result in large outbreaks," particularly if the virus is introduced into areas with communities with a high number of unvaccinated persons, the CDC researchers wrote in their report.
"These outbreaks demonstrate that unvaccinated persons place themselves and their communities at risk for measles," the researchers said.
Measles is a highly contagious illness that can lead to complications and death, but can be prevented by the MMR vaccine. An estimated 20 million people get measles each year worldwide, and Europe, a popular destination for U.S. travelers and an area where the virus continues to circulate, is most often the source of imported measles cases, according to the report.
Since 2000, when measles was declared eliminated in the United States in 2000, 60 cases have been reported yearly, on average. The highest numbers of cases reported since then were in 2008 and 2011, with 140 cases and 220 cases, respectively.
The CDC recommends vaccination for all children older than 6 months. Vaccine coverage in the United States is high, with 91 percent of U.S. children having been vaccinated, which limits the size of measles outbreaks, according to the report.
However, some states still have lower coverage levels. Unvaccinated children tend to be clustered in the same communities, increasing the risk for outbreaks.
Because the virus is likely to continue to be brought to the country, increases in the number of persons declining vaccination for themselves or their children might result in large-scale outbreaks, and threaten the elimination of measles in the United States, the researchers said.