Parts of Colorado are as vulnerable to measles as West Texas, where the highly contagious virus has infected hundreds of people and killed two.

Colorado has recorded more confirmed measles cases so far this year than it has in almost two decades. Rural pockets of the state are undervaccinated, and large counties have seen their protection erode since the pandemic.

The country as a whole has more than three times as many measles cases already this year as it did in all of 2024, and federal health officials are pushing vitamins as an alternative to vaccines.

“I think what’s clear is we’re going backward,” said Beth Carlton, leader of environmental and occupational health at the Colorado School of Public Health. “We’re creating conditions where we have the right ingredients for measles to spread.”

Statewide, about 94% of Colorado students in preschool through 12th grade were up to date on their measles shots in the 2023-24 school year, creating an imperfect but relatively solid wall around people with compromised immune systems and babies too young for the vaccine.

But that average conceals more-vulnerable pockets in the state: in two counties, one-quarter of children didn’t have their shots, and in eight schools, less than half did.

For the past 25 years, communities typically didn’t see any consequences from those low rates. Measles no longer circulated in the United States, and outbreaks popped up only when travelers brought the virus back. Although public health officials warned that declining vaccination rates could set the stage for the virus to start circulating widely again, Colorado reported only six people who had measles from 2014 to 2024.

Those days may be over. As of Thursday, measles had infected 1,046 people across the United States, and public health officials have started to worry that the outbreak could last into 2026.

Colorado has confirmed five measles cases since January, making this the first time since 1996 that the state had more than two in a single year. Those infected include an adult in Pueblo County, a Denver baby and an unidentified person who lives in the same household, a Denver adult, and an adult in Archuleta County.

State health officials also warned the public last week about two infected and potentially contagious travelers who recently visited Colorado: a person from another state who stayed at a Pueblo hotel May 9 and 10 and an international traveler who passed through Denver International Airport and stayed at a nearby hotel on May 13 and 14.

Most of the state’s confirmed measles patients got sick after visiting Mexico, although the Archuleta County adult hadn’t traveled, which raises concerns about missed cases.

Nationwide, uptake of routine vaccines still hadn’t rebounded from a pandemic dip as of 2023, and resistance to COVID-19 vaccines led to increased skepticism of shots that once had bipartisan support. State legislatures have considered expanding exemptions to school vaccination requirements or forbidding “discrimination” based on being unvaccinated, and a bill in Minnesota would have classified one type of vaccine technology as a “weapon of mass destruction.”

Health officials in the Trump administration have been lukewarm in endorsing the shots as a way to stem the current outbreak. Secretary of Health and Human Services Robert F. Kennedy Jr., who has promoted misinformation that vaccines cause autism and falsely claimed that vitamin A could prevent measles, framed vaccination as a personal choice, with no clear correct answer.

Two shots of the measles vaccine are about 97% effective in preventing infection and virtually eliminate the risk of serious complications, such as pneumonia and neurological damage.

At current vaccination rates, a series of simulations published by the Journal of the American Medical Association gave measles an 83% chance of reestablishing a permanent presence in the United States, with an estimated 850,000 cases and 2,500 deaths projected over the next 25 years.

But rates would have to rise only about 5% to prevent reestablishment and knock cases down to a few thousand in the next quarter century, the scientists running the simulation said.

Models never reflect reality perfectly, but seeing such a high risk from a simulation that used relatively conservative estimates is worrisome, Carlton said.

As outbreaks continue to spread in other states, and possibly in countries that have lost American aid for their health systems, Colorado communities will face an increased risk of imported cases that could turn into outbreaks if they land among undervaccinated people, she said.