Los Angeles County is one of the most vulnerable places in America when it comes to measles, largely because of the thousands of travelers arriving every day from countries with massive outbreaks, a new study says.
The study, published Thursday in the journal Lancet Infectious Diseases, put L.A. County near the top of a list of counties most likely to have someone come down with measles, second only to Cook County, Illinois.
So far this year, 764 Americans have been diagnosed with measles, the highest number of cases in more than two decades. The historic surge is due to an increasing number of measles outbreaks abroad as well as falling vaccination rates locally, which allow cases brought from elsewhere to proliferate, experts say.
Accordingly, researchers looked at vaccination rates in each county as well as the numbers of international travelers flying into each county from places experiencing major measles outbreaks, such as the Philippines and Ukraine. Thus far this year, according to the World Health Organization, 112,000 people have been diagnosed with measles outside the United States.
Rounding out the top five most at-risk U.S. counties were Miami-Dade in Florida, King in Washington and Queens in New York, which has — along with Brooklyn — been the epicenter of the nation’s biggest measles outbreak this year, with 466 cases, according to the city’s health department.
California’s San Mateo and San Diego counties also made the top 25.
Though California has shored up its vaccination rates in recent years, the volume of travelers landing at LAX, San Francisco International Airport — located in San Mateo County — and San Diego International Airport makes the state vulnerable regardless, said Johns Hopkins civil engineering associate professor Lauren Gardner, who is an author of the paper.
“The places, in particular in California — and also Cook County, which is where Chicago O’Hare is — are really high on the list mainly because of the sheer volume of travelers,” Gardner said. “It’s not just the fact that there are big airports, but those airports have a lot of incoming routes from countries having ongoing measles outbreaks.”
Indeed, this year alone, seven people with measles passed through Los Angeles International Airport, one through Long Beach Airport, and one through SFO, according to health officials.
And so far, the researchers’ rankings have matched up with reality. Almost all 25 counties on the list have already reported a measles case or are next to counties that have reported cases.
In Los Angeles County, there have been nine reported cases of measles this year. Measles began spreading here after people visited other countries with outbreaks, including Vietnam and Thailand. Those few cases probably did not expand into larger outbreaks, experts say, because of the state’s high vaccination rates.
The study ranks counties by how vulnerable they are to having a single case of measles pop up. It does not predict the size of a potential outbreak.
In 2015, after a major measles outbreak at Disneyland, California passed a law requiring all schoolchildren to be vaccinated unless they have a medical reason not to get their shots. The law has pushed up vaccination rates statewide.
Statewide, 43 people have been diagnosed with measles this year so far, according to state numbers released Thursday. Most of those are people who contracted the disease abroad, officials say.
The largest outbreak in the state has been limited to fewer than 20 people. An outbreak in San Francisco this year ended after only three people got sick. A Sacramento outbreak has also not spread beyond three people.
California is somewhat protected because a low number of people in the state are exempt from vaccines, Gardner said. Still, the state, and L.A. County in particular, remain vulnerable, she said.
“You could have another little town with higher exemption rates, but they’re not as well-connected to the rest of the world and they’re actually going to be at lower risk than Los Angeles,” she said.