Berlin has recorded 347 cases of measles since the start of the year, more than twice the number it had during all of 2014.
Officials believe the outbreak started with a child asylum seeker from Bosnia, because many subsequent infections among refugees were genetically linked.
"We consider this child to be the index case, because the measles virus this child had is identical to the ones that followed," said Dr. Dirk Werber of Berlin's state health office.
Measles is a highly contagious disease that sometimes results in severe complications, especially in very young children and adults. A vaccine widely introduced in the 1960s and 1970s has dramatically cut the number of infections worldwide. But outbreaks can occur when inoculation levels drop because of war, natural disasters or opposition to vaccination.
Werber said the outbreak in Berlin has spread to the city's non-refugee population, partly because immunization rates among over-45s are low. Younger adults, too, are at risk because many only received one shot instead of two, as now recommended.
"It is our impression that many of those affected weren't even aware that they lacked protection," he said. "These aren't people who oppose vaccination. They're people who just weren't aware of the problem."
Werber said at least two cases in Berlin have been linked to the United States. One involved a woman who developed symptoms in the United States before traveling to Berlin; a second involved a child who developed the infection after returning from the U.S.
While measles was declared eliminated in the U.S. in 2000, there continue to be cases carried in by travelers. Authorities have recorded more than 120 measles cases in the U.S. this year, most of them stemming from an outbreak that started at Disneyland.
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