Colorado allows three kinds of vaccine exemptions  religious, medical and those related to personal beliefs. 

(Photo: CPR/Megan Verlee)

For the first time, parents in Colorado can find out what portion of children at their kids’ schools have legally opted out of one or more of the vaccines required by state law, for things like measles, polio and tetanus. Viewed together, the data show that about 10 percent of schools in the state's 20 largest districts have opt-out rates high enough to threaten kids' health.

That's according to Chalkbeat Colorado, the online education news site, which obtained data from more than 1,000 schools. A new state law requires schools to provide vaccination information when asked. 

The schools in Chalkbeat's database educate more than three quarters of students in Colorado. 

Almost all states allow opt-outs for religious beliefs, and Colorado is one of 20 states that also allow families to opt out for philosophical reasons, according to the National Conference of State Legislatures.

But Ann Schimke, who led the project for Chalkbeat, says high opt-out rates worry public health experts, because having a significant portion of unvaccinated students threatens a school's "herd immunity." 

“You need a large majority of people in a given community or group vaccinated in order to ward off the spread of disease. Typically it’s in the 90-95 percent range,” Schimke says.

According to the data Chalkbeat got from schools, about 10 percent fall below that range.

Additionally, most of the schools in Chalkbeat's database don't have information on all students, as is required by law.

Chalkbeat's Nic Garcia worked with Schimke on the project. "Schools are very busy and sometimes they don't track down all the paperwork necessary. So there were some schools that were unable to provide us a complete picture of all students," Garcia says.

Among those schools: Mountain Phoenix Community School in Wheat Ridge, near Denver. It has vaccination data on just under half of enrolled students, according to Schimke. That troubles Charles Buchanan, who has a daughter at Mountain Phoenix. 

Buchanan told Schimke that knowing more about the vaccination rates at his daughter's school may cause him to switch schools. He says his daughter has a chronic condition that would make getting measles, for example, more detrimental to her health than it would be to an average child.

Compared with other states, Colorado's vaccination rates among children are very low. According to the Centers for Disease Control and Prevention, Colorado kindergartners have the lowest rate of vaccination against measles of any state, a cause for concern among some health experts -- and parents -- amidst the current measles outbreak in the U.S.