COVID-19 outbreaks are growing in Mesa County nursing homes and more people who’ve been at least partly vaccinated are getting sick enough across Colorado to be hospitalized as the more-contagious delta variant of the virus continues to spread.

While the Western Slope has been hit hardest, the variant originally identified in India already is moving across the state, accounting for three-quarters of new COVID-19 cases where the subtype is known.

A team from the Centers for Disease Control and Prevention arrived in Mesa County this week to investigate the spread of the delta variant. In a news release, state epidemiologist Dr. Rachel Herlihy said the state had requested the help from the CDC to learn “how and why” the delta variant was spreading in western Colorado.

She noted that the state had called on the CDC because of high hospitalization rates, following several nursing home outbreaks that included “breakthrough” cases, where vaccinated people were infected.

Mesa County came close to running out of hospital beds in recent weeks.

The CDC and the state health department are there to give Mesa County Public Health more resources to keep investigating, said Ginger Stringer, epidemiology response program manager at the Colorado Department of Public Health and Environment.

A team is going into long-term care facilities to see if the virus is behaving differently in recent outbreaks, which might require new strategies to keep it out, she said. Others are working on communicating the need to get vaccinated and to take precautions at large events.

“We have seen some really concerning trends,” Stringer said. “We don’t know if it’s abnormal, because we don’t know enough about the delta variant.”

CDC spokeswoman Jade Fulce said the agency sent an epidemiologist, an infection control specialist and a behavioral health scientist to help investigate the “potential for a high rate of vaccine breakthrough cases” in nursing homes, as well as the virus’s rapid spread. It’s too early to tell if Colorado’s situation fits a pattern for the delta variant, she said.

“Hence, we are providing onsite technical assistance,” she said.

During the week ending June 12, more than 90% of people hospitalized statewide with a confirmed case of COVID-19 — some of whom may have sought care for another reason — hadn’t received a dose of the vaccine, according to the state health department.

That shows the odds of getting sick are significantly lower after vaccination, but it also suggests a higher rate of severe breakthrough cases than the state was seeing a few weeks ago.

Fewer than 2% of Colorado’s confirmed COVID-19 cases between mid-January and early June were in people who were fully vaccinated, and most of those had mild symptoms that didn’t require hospitalization, according to the state health department.

Data from the United Kingdom suggests the Pfizer vaccine is slightly less effective against the delta variant than it was against previous versions of the virus, with significantly less protection for people who have only received one dose. Little data is available about the Moderna and Johnson & Johnson vaccines, which aren’t widely used in the U.K.

The state also announced that nursing homes and other long-term care facilities in counties where the delta variant is spreading will have to test all unvaccinated residents and staff using a highly reliable test that looks for the virus’s genetic material twice weekly. Unvaccinated staff also will have to take a slightly less-accurate rapid test before beginning each shift.

One in seven currently active COVID-19 outbreaks in Colorado assisted living facilities and nursing homes is in Mesa County, far exceeding the area’s share of such facilities. The eight outbreaks in Mesa County involve 37 cases among residents, 47 staff cases and five deaths — more than one-fifth of all cases and more than half of deaths linked to long-term care facilities with current outbreaks.

The delta variant now accounts for the majority of cases in Colorado and has been found in 28 counties. It’s about 50% more contagious than the alpha variant, which previously dominated in Colorado and was itself more contagious than the original version of the virus. Early data from Scotland suggests people infected with delta were more likely to be hospitalized than those who had alpha, but scientists are still trying to verify that.

As of Tuesday afternoon, the state had identified 534 cases of the delta variant in Colorado, with more than half found in Mesa County. The state doesn’t run complex genetic analysis on every positive COVID-19 test, though, so the actual number of delta infections may be higher.

The counties with the most confirmed cases of the delta variant as of Tuesday were:

• Mesa County: 288
• El Paso County: 103
• Garfield County: 27
• Douglas County: 20
• Moffat County: 18
• Arapahoe County: 12
• Boulder County: 12
• Adams County: 10

The Colorado Department of Public Health and Environment’s dashboard showed about 75% of statewide virus samples sequenced in the week of June 13 contained the delta variant.