SAN FRANCISCO (KPIX 5) — According to a new study, San Mateo County is one of the most vulnerable places in the country to measles, ranking 18th nationwide.

The reason why San Mateo County ranks so high is because of the San Francisco International Airport. Many of the measles cases in California have involved international travelers or someone who has come in contact with people that have traveled abroad.

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California has seen the highest number of measles cases since 1993. According to the The Lancet Infectious Diseases Journal, 9 of the 44 cases seen in the Golden State have been found in the Bay Area.

“It was something that was ancient history but I’m surprised it’s made a comeback,” said Amir Torkaman, a San Mateo County resident.

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The county is ranked 18th most at risk by Lancet, but it has only seen four cases so far. The counties that have reported the most number of measles cases are home to international airports, with O’Hare International Airport in Cook County, Illinois topping the list.

The San Mateo County Health Department says all four of the cases they recently discovered have a direct tie to the airport. Doctors say if you have any concerns about possibly catching measles, get vaccinated now.

Our partners at KCBS radio spoke to Doctor William Shaffner from Vanderbilt University. “If you have any concerns, go get vaccinated. Even if you’re not sure if you’ve been vaccinated or not, you won’t be harmed but protected,” said Shaffner.

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The San Mateo County Health Department has looked into 60 suspected cases and hundreds of contacts including Uber and Lyft drivers and at large companies and schools.