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We Can Predict Where Measles Will Happen. Why Don’t We?

At the end of February, a team of researchers submitted a paper to the medical journal The Lancet that predicted the top 25 U.S. counties most at risk for measles in 2019. Now, almost halfway through what is on track to be the nation’s worst year for measles outbreaks since 1992, their study is looking rather prescient. Of the counties named in the paper, published in May, 14 have had cases of measles. At least 12 counties on the list are adjacent to counties that have ended up with measles cases this year. This includes two of the counties in the largest ongoing outbreak: Nearly 500 confirmed cases in Queens and Kings counties, New York.

The 25 highest-risk areas for measles

The U.S. counties most vulnerable to measles ranked by researchers’ calculations of how likely they were to have an outbreak in 2019, and whether cases have been reported this year

Rank County State 2019 cases
1 Cook Illinois
2 Los Angeles California
3 Miami-Dade Florida
4 Queens New-York
5 King Washington
6 Maricopa Arizona
7 Broward Florida
8 Clark Nevada
9 Harris Texas
10 Honolulu Hawaii
11 Wayne Michigan
12 Tarrant Texas
13 Multnomah Oregon
14 Orange Florida
15 Essex New Jersey
16 Denver Colorado
17 Hillsborough Florida
18 San Mateo California
19 Salt Lake Utah
20 Suffolk Massachusetts
21 Clayton Georgia
22 Travis Texas
23 Hennepin Minnesota
24 Loudoun Virginia
25 San Diego California

*Relative risk is the probability of an event occurring in an exposed group, divided by the probability of it happening in a non-exposed group.

Sources: The Lancet, The New York Times, State Departments of Health, The Washington Post, Reuters, Centers for Disease Control And Prevention

But that was not what’s special about this forecast. In the 19 years since measles was eliminated1in the U.S., experts say domestic outbreaks have followed a reliable pattern that should be easily traceable in epidemiological data. Everybody knows the pattern exists, so it’s no surprise that someone could use it to make predictions about where measles is most likely to turn up. Instead, experts say, the county list is notable because of who made it. It came from independent researchers, and not the government, which collects the data.

There are basically two things you need to make a measles outbreak in the U.S. The first is a local community with a low vaccination rate. Although 91 percent of Americans nationally are vaccinated against measles, isolated pockets can have much lower rates — 70 percent, 50 percent, or even less. Insular communities connected around shared culture, religion or a single school provide a place where measles can incubate and spread, said Peter Hotez, professor of pediatrics and molecular virology at Baylor University. If a measles outbreak is a fire, you can think of this as the tinder.

But you still need a lit match, and that’s where international travel comes in.

Measles outbreaks no longer start on their own anywhere in the Americas. The cases we have have to come from somewhere else. “It’s usually someone going to Europe, the Philippines, Israel,” said Daniel Salmon, professor at Johns Hopkins Bloomberg School of Public Health. That is, Americans travel abroad and we’re exposed to measles in countries where the disease is still endemic. If we are not vaccinated and return home to a community with a low overall vaccination rate, that’s when the fires get lit.

That predictive study in the Lancet arrived at its list of at-risk counties by cross-referencing communities with low vaccination rates and high rates of kindergarten vaccine exemptions with communities whose airline hubs have frequent flights to countries with ongoing measles outbreaks. In the case of the New York outbreak, the data connected a close-knit Orthodox Jewish community to Israel, where measles is endemic, infecting more than 3,500 people there between March 2018 and February 2019.

In response, New York has mandated vaccination for everyone living in a few highly affected ZIP codes and threatened to fine people who don’t get the shot. Vaccine mandates in the U.S. are normally tied to school attendance — not travel, said Saad Omer, incoming director of the Yale Institute of Global Health. There are legal and constitutional frameworks that give states the right to mandate vaccines in public schools, he told me. But the same doesn’t exist for requiring someone to get vaccinated before they travel to Israel or the Ukraine or any other country where measles outbreaks are common.

Meanwhile, there’s not been much effort made, on a national scale, to publicly predict which low-vaccination communities in the U.S. are most at risk, which makes it hard to target prevention efforts in those places. Even data on where measles outbreaks do happen in this country is often vague and scattered across public health agencies in multiple states. For instance, the Centers for Disease Control and Prevention says it has data breaking down confirmed cases of measles by county and ZIP code, but it doesn’t publish it. Sahotra Sarkar, a professor of philosophy and integrative biology at the University of Texas at Austin, one of the lead authors on the 2019 study, said if the agency did provide that data, he and is colleagues could use it to refine their maps.

The CDC told me they have the counties and ZIP codes where cases of measles have happened and information on which countries contributed the spark — but that the information is not compiled or easily available in one place. They declined to comment on why. That’s a problem, Hotez told me. More-detailed maps that track pockets of low-vaccination rates, vaccine exemptions and the countries each outbreak comes from could be used to help stave off outbreaks, something that’s becoming particularly important as outbreaks increase and the U.S. faces losing its status as a country where measles has been eliminated. In Hotez’s view, predictions like the ones from Sarkar should be coming from the CDC annually. But they’re not.

From ABC News:

Number of measles cases in 2019 surpass 25-year record


  1. Meaning measles is not circulating continuously in the US population as a whole. When the disease pops up, it can be traced to importation from another country.

Maggie Koerth was a senior reporter for FiveThirtyEight.