An unvaccinated Adams County child who tested positive for measles is tied to a case from Broomfield High School last month, leading state health officials to declare an outbreak.

Colorado has four measles cases to date, amid a national outbreak. The other cases are in Broomfield and Arapahoe counties.

The latest case, state officials said, was identified at Broomfield Heights Middle School.

State health officials on Wednesday said they are investigating other exposures.

The public is encouraged to watch for symptoms up to 21 days after exposure.

Measles is a highly contagious and preventative respiratory illness. The disease is transmitted through direct contact with infected droplets or airborne when an individual breathes, coughs or sneezes.

Symptoms typically emerge within two weeks of exposure. The telltale sign of a measles infection is a spotty red rash. In rare cases, measles can cause swelling of the brain and death.

A highly effective vaccination campaign is credited with eradicating measles in 2000. Before the vaccine, an estimated 48,000 Americans were hospitalized with measles and 500 died each year.

The four measles cases reported so far this year exceed the total confirmed in Colorado since 2017.

After a confirmed measles case last week at the high school, officials requested more than two dozen unvaccinated students not to attend class.

“In order to attend school, you must present proof of measles immunity,” Broomfield High School Principal Ginger Ramsey said in an email last week.

The high school is working on academic support for these students, Randy Barber, a Boulder Valley spokesperson, has said.

The high school has about 1,671 students, according to the Colorado Department of Education.

It’s unclear what steps are being taken at the middle school.

Barber could not be immediately reached for comment.

Public health officials have said that the high school student had not traveled outside the state or had a known exposure, according to state health officials.

“The lack of a clear source of infection suggests that unidentified measles cases may be occurring in or traveling through the area,” state officials have said.

Dr. Michelle Barron, senior medical director of infection prevention and control for UCHealth, has warned about the risk of out-of-state travelers who may be unvaccinated.

“Colorado’s big risk right now is it being imported from somebody visiting,” Barron has said.

About 93.3% of students in Colorado were vaccinated for measles, mumps and rubella last school year, state vaccination shows.

Colorado recorded 36 measles cases last year — more than the previous decade combined — with roughly one in four associated with an out-of-state traveler who flew while infectious, according to the Colorado Department of Public Health and Environment.

The positive cases in Colorado come as the United States has seen an unprecedented rise in the number of cases.

As of Feb. 27, there were 1,136 confirmed measles cases, with roughly 90% associated with an outbreak, according to the Centers for Disease Control and Prevention.