After state officials confirmed a measles case last week at Broomfield High School, 26 students are not allowed to attend class, the district said Monday.

On Friday, school officials requested students who are not vaccinated against measles to not attend class.

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

It is unclear what support will be offered to students who cannot attend class. It is also uncertain when unvaccinated students will be allowed to return.

“The school is currently working on academic supports and will be communicating that information out to families by mid-week,” Randy Barber, a Boulder Valley spokesperson, said in an email to The Denver Gazette.

On Friday the number of excluded was 42, but as district and the public health staff have worked to collect information on student vaccination status half were taken off the list, Barber said.

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

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.

Public health officials confirmed a measles case last week with an unvaccinated student. The 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.

To date, Colorado has had two measles cases. The other is in Arapahoe County.

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.

Measles was considered eradicated in 2000, which health officials credited to a highly affective vaccination campaign.

Before a vaccine was developed, an estimated 48,000 Americans were hospitalized with measles and 500 died each year.

About 93.3% of students in Colorado were vaccinated for measles, mumps and rubella last school year, state vaccination data shows. To be protective in a community, 95% or more of students should be vaccinated against measles.

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 in 26 states with roughly 90% associated with an outbreak, according to the Centers for Disease Control and Prevention.