Amid a record number of measles cases nationwide, about 24,000 Colorado kids went back to school this fall with personal or religious exemptions from the vaccination that protects against the highly contagious disease.

Public health experts say Colorado is particularly susceptible to a measles outbreak because of low immunization rates, with the state’s lenient exemption policies partly to blame. But efforts to tighten rules for non-medical exemptions, including a bill that died in the legislature last spring, have sparked fierce opposition.

A new report notes that Colorado’s exemption rate among kindergarteners ticked up slightly last year while the share of kindergarteners fully immunized against measles fell to 87%, from 89% the previous year.

Last year, 2.75% of more than 864,000 Colorado schoolchildren had religious or personal belief exemptions from the vaccine that protects against measles, mumps and rubella, commonly called MMR. Another 0.1% of students had exemptions for medical reasons. Meanwhile, the MMR vaccination status of nearly 7% of students was unknown, either because their shot records were incomplete or never submitted to their schools.

The report, from Children’s Hospital Colorado and the Colorado Children’s Immunization Coalition, also found growing resistance from schools to publicly reporting their immunization and exemption rates.

While state rules require schools to provide this information to the health department annually, only 89% of schools reported it last year, compared with 94% the year before. The state publishes school immunization and exemption rates in a searchable online database to help parents and members of the public gauge local disease risk.

There has been one confirmed case of measles in Colorado this year, and state health department officials say they have investigated at least 64 other reports of people suspected of having measles, but all were ruled out.

The number of measles cases has ballooned across the country in 2019, with outbreaks in several states, including New York, Washington, Oregon, and Texas. Nationwide, there have been 1,241 cases in 31 states. In contrast, there were 372 measles cases nationwide in 2018 and just 63 in 2010, according to the Centers for Disease Control and Prevention.

About 95% of people in a population must be vaccinated to prevent a disease like measles — a concept known as herd immunity. Colorado’s relatively low measles vaccination rate illustrates the state’s vulnerability on a big-picture level, but the risk of an outbreak varies widely by community.

Pockets of unimmunized students can be found in plenty of Colorado schools, including charters with Montessori or Waldorf-inspired curriculums, district-run enrichment programs for homeschooled children, and some alternative schools. At least 67 schools across the state had measles vaccination rates under 80% last year, according to data from the state health department.

The searchable chart below shows 2018-19 MMR vaccination and “personal belief” exemption rates for Colorado schools. The chart doesn’t show the percentage of students with religious or medical exemptions, the percentage of students “in process,” or the percentage who have turned in no vaccination or exemption paperwork at all. The data comes from the Colorado Department Of Public Health and Environment.