Three children visiting Colorado have been hospitalized with measles, leading health officials to warn people who traveled through Denver International Airport earlier this week that they are at risk for the highly contagious disease.

The children tested positive after traveling to a country with an ongoing measles outbreak. They did not have the MMR — or measles, mumps and rubella — vaccine, according to a news release from Tri-County Health Department, which covers Adams, Arapahoe and Douglas counties.

The Centers for Disease Control and Prevention considers three or more cases of measles “linked in time and place” to be an outbreak. However, Tri-County Health spokesman Gary Sky said the department doesn’t consider this to be an outbreak because the patients are related.

Health officials said individuals who visited these locations may have been exposed to measles:

Denver International Airport between 1:15 and 5:45 p.m. Dec. 11

Children’s Hospital Colorado’s Anschutz Campus Emergency Department between 1 and 7:30 p.m. Dec. 12

Local health officials have not said where the family was traveling from. But the news of the measles cases in Colorado comes the same day that health officials in California warned about exposure from patients who traveled through Los Angeles International Airport.

It’s unclear how many people are at risk of exposure.

Officials at Denver International Airport said they do not know how many people potentially came in contact with the children. Roughly 179,000 people passed through the airport via departing, arriving or connecting flights on Dec. 11, said airport spokeswoman Emily Williams.

Health officials are contacting people who are believed to be at risk for measles, including those who visited Children’s Hospital on Dec. 12. The Tri-County Health Department will likely contact “well over 100” people in its investigation, said Dr. Bernadette Albanese, a medical epidemiologist.

“We’re doing this investigation for a reason, and that reason is precisely to prevent secondary spread — and having a non-ideal vaccination rate in Colorado isn’t helping matters,” she said.

There is no ongoing risk of exposure at these two locations, however, travelers should be on the lookout for measles symptoms, which can develop seven to 21 days after contact, the news release said.

Measles has various symptoms including high fever, cough, runny nose, watery eyes and a rash. The illness can lead to pneumonia and swelling of the brain, according to the Centers for Disease Control and Prevention.

Measles is highly contagious and up to 90% of people close to a person with the illness become infected if they are not immune, according to the CDC.

Representatives of the Colorado Department of Public Health and Environment and Children’s Hospital Colorado declined to discuss the measles cases and deferred questions to Tri-County Health Department.

Several measles outbreaks have occurred across the United States this year, but until now there was only one case reported in Colorado. In January, a Denver resident was placed in isolation and treated for the respiratory illness.

But health experts have warned that Colorado’s low vaccination rate makes communities here vulnerable to an outbreak. The immunization rate for the MMR shot was 87.4% during the 2018-19 school year, meaning the state doesn’t meet the threshold needed to protect a community from a measles outbreak.

The state’s low vaccination rate has come under scrutiny this year and a bill to make it harder to opt out of such shots was debated by legislators before it failed. Gov. Jared Polis has said he’s “pro-choice” when it comes to vaccinations. He said believes the solution to raise the low immunization rate is through education and access rather than eliminating nonmedical exemptions.

If a person has symptoms that could be measles they should call their doctor’s office or a hospital first, the news release said.

Subscribe to bi-weekly newsletter to get health news sent straight to your inbox.

Updated 3 p.m. Dec. 16, 2019 Due to incorrect information from a health official, this story originally mischaracterized the measles cases at Denver International Airport as an outbreak.