Colorado public health leaders responded cautiously to news that the rapidly spreading delta variant may pose an even greater threat than believed, continuing to preach the effectiveness of COVID-19 vaccines as some metro agencies recommended that inoculated residents resume wearing masks in indoor public spaces.

The Tri-County Health Department, Jefferson County Public Health and Broomfield Public Health and Environment issued statements at the end of the week echoing the new guidance from the Centers for Disease Control and Prevention that recommended vaccinated people mask up in counties with significant transmission of the coronavirus.

The CDC guidance applies to most Colorado counties, including the entire Denver metro area and Front Range.

“Many health departments were waiting on the state,” said John Douglas, executive director of Tri-County Health Department, which serves Adams, Arapahoe and Douglas counties. “We felt like we couldn’t wait any longer.”

Though the Colorado Department of Public Health and Environment hasn’t taken a public stance on the new CDC mask guidance, the Polis administration sent a statement to The Denver Post on Friday evening saying that it was “updating state guidance to align with the new CDC guidance.”

There was no specific reference to the mask recommendation, and a spokesman for Gov. Jared Polis did not respond to requests for elaboration.

Later Friday night, the state health department announced it had updated its back-to-school recommendations based on the portion of the CDC’s new guidance that calls for all students, staff and visitors to K-12 schools to wear masks inside. The state’s new guidance recommends Colorado districts “consider either mask requirements for all unvaccinated individuals or all individuals in schools.”

Responding to questions from The Post, a representative of the state health department said in an email that “we do not plan a new statewide mask mandate at this juncture,” but did not address whether Colorado will follow the CDC in recommending vaccinated people in counties with high transmission wear masks in indoor public places.

“It’s fully appropriate for local health agencies to be leading the way given the vast variability of disease transmission and vaccination uptake across the state with some counties with vaccination rates in the forties and others in the eighties,” Conor Cahill, a spokesman for the governor, said in an email.

Denver officials noted that the city’s current public health order requiring unvaccinated people to wear masks in certain settings expires over the weekend. The city plans to issue an unspecified new public health order Monday.

“We respect this guidance and encourage individuals and businesses to make the decision about wearing a face covering that is best for them,” the Denver Department of Public Health and Environment said of the new CDC recommendations in the agency’s weekly COVID-19 update report.

The CDC’s change in guidance for face coverings was driven in part by new data that shows that vaccinated people infected by the delta variant — the so-called “breakthrough” cases — may be able to spread the virus to others.

“The largest concern that I have, that truly keeps me up at night, is that delta is so infectious that there is an increase likelihood of breakthrough cases among people who are fully vaccinated,” said Dawn Comstock, executive director of Jefferson County Public Health.

Public health officials emphasize that the COVID-19 vaccines remain very effective against severe illness and death. Breakthrough infections are relatively rare.

But the CDC’s findings mean “we have to take all of this seriously and be careful about how we approach it,” said Alex Huffman, an aerosol scientist at the University of Denver. “Masks do a great job of helping to prevent the spread of this virus.”

The public local health three agencies stopped short of requiring masks in indoor public spaces, although Comstock and Douglas both didn’t rule out the possibility of a mandate if their communities saw a significant rise in cases and hospitalizations.

Douglas said it wasn’t the right time to require masks.

“We do have vaccines and we do have a public in many parts of the state that are wary of public health orders,” he said, adding, “Honestly, if we would be mandating anything, I would encourage employers to mandate the vaccine.”

There already are efforts underway to get more Coloradans vaccinated. UCHealth and Denver Health said this week that they are mandating the shots. Polis announced Friday that state employees will have to undergo twice-weekly testing and wear masks in indoor public areas unless they get vaccinated.