(CNN) A new study ranks the US counties at highest risk of a measles outbreak, with the areas surrounding Chicago, Los Angeles and Miami topping the list.

The authors say places like these should be kept under close surveillance because they “could serve as the fulcrum of continuous importation of the measles virus into the USA.” Their paper was published Thursday in the journal The Lancet Infectious Diseases.

“What we’re trying to do is identify what are the areas that are most likely to be affected by this [anti-vaccination] movement that’s happening,” said study co-author Lauren Gardner, an associate professor in the Department of Civil Engineering at Johns Hopkins University.

In order to rank the counties at greatest risk, the authors didn’t even have to consider the locations of actual measles cases in the United States. Instead, they looked at factors including nonmedical vaccine exemptions, international air travel and the incidence of measles in countries people are traveling from.

“These outbreaks can only happen if someone brings measles into the country and there’s a low vaccination rate in a population,” Gardner explained.

“The places where measles outbreaks are happening this year were the ones that we ranked really highly in our risk model,” which had largely been completed by February, she added.

According to the paper, of at least 45 counties reporting measles cases this year, 30 were among or adjacent to the researchers’ top 25, including outbreaks in states like New York, Washington and Oregon.

This year, the United States has seen the highest number of confirmed cases since measles was declared eliminated in the country in 2000: now more than 760 cases, according to the US Centers for Disease Control and Prevention.

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