Colorado, Utah, and Wyoming all came in below the national vaccination average of 91.5 percent by a few percentage points.
Sean O’Leary, pediatric infectious diseases specialist at Children’s Hospital of Colorado, said that may not seem like much of a difference, but added, "when you get measles vaccination rates much below 90 percent in a given area, you are really vulnerable to an outbreak."
He said public health experts pay most attention to smaller pockets, like certain neighborhoods or schools, where vaccination rates are very low.
"Those are the places where outbreaks start," said O'Leary.
Colorado, Utah, and Idaho all allow parents to opt out of mandated childhood vaccinations for religious or philosophical reasons.
Colorado is one of 10 states that has reported measles cases to the CDC so far this year.
"In general," said O’Leary, "the states that have the easier exemption policies have higher rates of vaccine-preventable diseases."
Measles is a highly contagious disease. In 2017 the virus was responsible for 110,000 deaths worldwide. Most of the victims were under five.
This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Boise State Public Radio in Idaho, KUER in Salt Lake City and KRCC and KUNC in Colorado.