Denver’s COVID-19 risk level has been raised to medium by the Centers for Disease Control and Prevention, the city announced Friday afternoon, amid a recent, statewide uptick in cases driven by new and increasingly transmissible versions of the virus. 

Colorado reported more than 2,300 COVID-19 cases on Thursday, the most in a single day since early February. One hundred and sixteen people are hospitalized with COVID-19 statewide, the highest number since the third week of March but still a fraction of the total from January. State and local officials, though, maintain that high levels of immunity among the population should insulate Colorado from the sort of surges that it experienced in late 2021 and early 2022.

“While the current surge is not expected to be nearly as large as the Omicron surge earlier this year,” the Denver Department of Public Health and Environment said in a Friday afternoon news release, “it’s a good opportunity to remind our community how to stay safe, protected and prepared for COVID-19.”

Denver’s per-100,000 resident incident rate has increased from a low of roughly 50 in mid-March to more than 170 as of last week, when data was last updated in full. 

Denver – and the rest of the state’s counties – had been rated by the CDC as in the green, or low, for COVID-19 as of Thursday morning. But Rachel Herlihy, the state’s epidemiologist, told a gathering of medical experts Thursday that she expected some areas to be upgraded, given the increase in cases statewide.

As of Friday afternoon, Boulder and Mineral counties have both been placed in the yellow, or medium, category. Under the designation, the CDC recommends high-risk individuals consider wearing masks and taking other precautions and that the general public ensure they’re fully vaccinated and get tested if they feel sick.

The rise in cases statewide comes as a new subvariant of omicron – the strain that caused a record-breaking surge in cases in January and December – has risen in prominence in Colorado. BA.2.12.1, as it’s known, is increasingly replacing BA.2, another omicron subvariant, as the dominant strain in Colorado. BA.2.12.1 is more transmissible than both BA.2 and the original strain of omicron.

According to new modeling from a team of Colorado academics, COVID-19 hospitalizations could increase to at least 500 – and potentially hit 800 – by mid-June. Those figures are dependent on BA.2.12.1’s ability to escape immunity from previous infections, vaccination or both. While the state had high levels of immunity in February, those numbers have fallen, and BA.2.12.1’s ability to escape that protection is a further complication.

“We estimate that immunity to infection with BA.2.12.1 is approximately 65-70% in the Colorado population, assuming BA.2.12.1 has high immune escape,” the modelers wrote. “Immunity to severe disease is well above 80% in the Colorado population overall and somewhat higher among those 65+.”

There are other variables as well. Another substrain of omicron, BA.4, was identified in Denver last week from a sample taken in late April. BA.4 has contributed to surging cases in South Africa. A spokesman for the state health department told The Denver Gazette on Thursday that BA.4 doesn’t appear to cause more severe disease, but that it could lead to more cases than was projected previously.

Herlihy told the medical group Thursday that the increasing hospitalizations could bring strain and challenges to Colorado’s hospitals. But even at the higher end of the projections – 800 hospitalized COVID-19 patients in the next four to six weeks – Colorado would still be at roughly half the number of hospital patients as it had during the peak of omicron in January.