The number of new COVID-19 cases in Colorado spiked in the days around Christmas, the latest state health department data show, with the biggest single-day new infections reported since the beginning of the global coronavirus pandemic.

Over the holiday weekend, the Colorado Department of Public Health & Environment reported more than 10,000 new cases, signaling again the rapidly spreading nature of the most recent omicron variant of the virus.

At the same time, the number of vaccinations administered over the weekend dropped to the lowest point in months.

Some of the state’s mountain, ski-country counties have seen the biggest surge in recent weeks, with Eagle, Pitkin and Summit counties leading the state in new infections per 100,000 residents, each hitting new single-day infection highs Monday.

The same trend has emerged in the more populous parts of the state as well, with Denver, Arapahoe and Douglas counties also reaching all-time high numbers of new infections per 100,000 residents.

In the last week, Summit County recorded about 2,401 cases per 100,000 people, or 745 total new cases, a Monday news release from the county said.

Testing demands there have also surged, with three community testing sites collecting between 500 and 1,000 tests each day, according to the release.

“These high case numbers are impacting Summit County businesses and critical services due to staffing shortages,” the release said.

For most of the state, the trajectory of new cases is steeply rising.

COVID-19 hospitalizations in the state had been declining in the past couple weeks, but have leveled out, and with so many new cases, could soon be on the rise again.

The spread of coronavirus in El Paso County had been falling for the first two weeks of December, but that trend seems to be reversing as the highly contagious omicron variant drives more infections statewide.

The average number of new cases in the county dropped to 203 per 100,000 people on average over seven days on Dec. 19, a low not seen since August following a long high-plateau of cases, El Paso County Public Health data show. Since Dec. 19, the number of people testing positive for the virus has climbed, and as of Sunday the county was averaging 283 new cases per 100,000 people over seven days, with positive tests for nearly 11% of those tested after falling to close to 7% in mid-December. 

“After seeing a pretty sustained decline, it’s concerning that we might be starting to go up again,” said Michelle Beyrle, El Paso County Public Health spokeswoman. 

State officials shared data last week showing the variant is widespread, with almost half of all positive COVID-19 samples sequenced in the state lab had a genetic marker for the omicron variant, state epidemiologist Rachel Herlihy told reporters last Wednesday. The delta variant of the virus took about eight weeks to account for more than half of all cases, a milestone the omicron variant passed in about half the time.