Since the beginning of 2015, Berlin has recorded 347 cases of measles - more than twice the number it had during all of 2014.

Dr. Dirk Werber of Berlin's state health office said on Friday that at least two cases in Berlin have been linked to the United States. One involved a woman who developed symptoms in the US before traveling to Berlin and a second involved a child who developed the infection after returning from America.

Despite declaring that measles had been eliminated from the United States in 2000, public health officials in California are also currently battling a measles outbreak. Latest reports suggest that more than 120 people have been infected in the country, most of them originated from an outbreak that started at Disneyland.

Refugee index case

German officials also said on Friday that they believe the current outbreak began with a child asylum seeker from Bosnia, as many of the following 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," Werber said.

Werber said the outbreak in Germany's capital has spread to Berlin's non-refugee population, partly because immunization rates among over-45s are low.

Younger adults, too, are at risk because many only received one vaccination instead of the recommended two.

"It is our impression that many of those affected weren't even aware that they lacked protection," Werber said. "These aren't people who oppose vaccination. They're people who just weren't aware of the problem."

Many experts believe that efforts to encourage parents to have their children vaccinated were derailed by incorrect science, which suggested the combined measles, mumps and rubella (MMR) vaccine could cause autism.

The allegation was made in 1998 by Andrew Wakefield, a British medical practitioner, whose study has since been withdrawn.

Largest outbreak since 2001

In Germany, the immunization rate currently lies at around 90 percent - some 5 percent below the goal set by Germany and other European countries to eradicate the disease by 2015.

In 2013, the German Health Minister at the time, Daniel Bahr, even considered mandatory vaccination in a bid to wipe out the highly contagious disease.

Earlier this month, Germany's Robert Koch Institute (RKI) said the country is facing its largest outbreak of measles since the Infection Protection Act was introduced in 2001.

The airborne disease which is easily spread by coughs and sneezes can sometimes result in severe complications, especially in very young children and adults.

ksb/sms (AP, dpa)