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Measles cases have reached a 25-year record. Getty Images

The measles has made a historic comeback this year, with new cases popping up across the country week after week.

Since January 1, there have been 839 reported cases of the measles in 23 states, according to the Centers for Disease Control and Prevention (CDC).

This is the largest number of measles cases the United States, has seen since 1994, and from the looks of it, measles activity doesn’t look to be slowing down anytime soon.

Nervous parents, medical experts and people who are immunocompromised have all been wondering where exactly the latest outbreak will appear.

Now researchers from Johns Hopkins University and the University of Texas at Austin have identified the top 25 counties most at risk to experience a measles outbreak next. The predictions are based on multiple contributing factors, including a region’s vaccination rates, proximity to an international travel hub, along with the county’s population and size.

The analysis, which was published in the journal The Lancet Infectious Diseases last week, found that counties containing big metropolitan areas were the most at risk. The top three counties at risk were Cook County, Illinois, which contains Chicago, followed by Los Angeles County and Miami-Dade County in Florida.

Health experts hope the findings can help public health officials in the at-risk counties prepare for what’s coming and focus on their surveillance and vaccination efforts.

“It’s important to identify at-risk counties ahead of time so public health officials can fortify the population by encouraging vaccination to take rates to the highest level possible,” Dr. Amesh Adalja, a board-certified infectious disease specialist and a senior scholar at the Johns Hopkins Center for Health Security, told Healthline. “Identification [of the at-risk counties] also helps for resource planning at health departments that will be called into action if an outbreak occurs.”

To determine which counties are most likely to be struck by the virus, the researchers conducted a risk analysis by looking at each counties’ vaccination rates, the population size, the location of global measles outbreaks, and the amount of international travelers — especially from the afflicted countries — that typically visit.

The researchers found that their analysis accurately predicted the areas in New York, Oregon, and Washington, which have already been struck by major measles outbreaks earlier this year.

According to the researchers, all of the counties listed in the predictive analysis that have not yet had an outbreak this year are either next to a county that’s experienced an outbreak or located near an international airport.

The 25 counties most at-risk are: Cook, Illinois; Los Angeles, California; Miami-Dade, Florida; Queens, New York; King, Washington; Maricopa, Arizona; Broward, Florida; Clark, Nevada; Harris, Texas; Honolulu, Hawaii; Wayne, Michigan; Tarrant, Texas; Multnomah, Oregon; Orange, Florida; Essex, New Jersey; Denver, Colorado; Hillsborough, Florida; San Mateo, California; Salt Lake, Utah; Suffolk, Massachusetts; Clayton, Georgia; Travis, Texas; Hennepin, Minnesota; Loudoun, Virginia; San Diego, California.

From May 6 through May 11, a man who tested positive for the measles potentially exposed hundreds of people to the virus in King County, Washington, suggesting cases may soon multiply in the area, as predicted.

On top of identifying which U.S. countries were most at risk for a measles outbreak, the researchers also identified which countries have experienced notable outbreaks this year and pose the greatest risk to the United States.

They include China, India, Japan, Mexico, Philippines, Thailand, and Ukraine.

According to the research team, the proximity of major international airports to at-risk areas could play an essential role in continuing to welcome the measles virus back into the United States

Since 2000, when the measles was eliminated from the United States, every measles outbreak has resulted from a case that was brought back from overseas.

In fact, this is how the outbreaks took off in New York earlier this year.

“Both Rockland County and Brooklyn are near a major international hub (JFK airport) and have large cohorts of unvaccinated individuals. An infected traveler from Israel, where an outbreak is occurring, was all that was needed to start outbreaks in this ripe area,” Adalja explained.

The researchers hope that local health officials can increase surveillance efforts in the U.S. counties that see a lot of international travelers come in from these countries.

The measles is extremely contagious — nearly 90 percent of unvaccinated individuals will get sick if exposed to the virus.

The easiest and most effective way to avoid an outbreak is to encourage more people to get vaccinated.

If these counties were able to get their vaccinations rates up to 95 percent it would help prevent the virus from spreading within the community, health experts believe.

“The most important thing people in these counties can do is to make sure they are vaccinated along with their children,” says Dr. Beulette Hooks, a family physician and chair of the American Academy of Family Physician’s Commission on Health of the Public and Science.

According to Hooks, every child, minus those who have severe health conditions, starting at age 1, needs to get the first dose of the measles-mumps-rubella (MMR) vaccine and the second dose at age 4 before starting school.

If measles cases do appear in the area, Hooks recommends that unvaccinated people avoid public areas — especially where there may be people from the foreign countries experiencing outbreaks.

In addition, if an outbreak is imminent, health officials should consider enacting emergency declarations that either modify school attendance policies or order mandatory vaccinations to prevent outbreaks from worsening, Adalja suggested.

“There are two critical steps to stopping the spread. First, isolate the people who have the virus. Second, vaccinate, vaccinate, vaccinate,” Dr. David Cope, a family physician from Bountiful, UT, said.

As long as the virus can travel from person to person, these devastating outbreaks will continue, and even more counties will soon be at risk.

New research identified the top 25 counties most likely to experience a measles outbreak next. The analysis suggests that Cook County, Illinois, Los Angeles County, and Miami-Dade County in Florida have the highest risk of being struck by the measles. In order to avoid potential outbreaks, those who have the virus need to be isolated and nearly everyone else needs to get vaccinated, health experts say.