COVID-19 hospitalizations in Colorado have increased for the first time after weeks of welcome declines, with the reversal fueled by the increased presence of sub-strains of the omicron variant.

Hospitalizations, as well as case and positivity rates, have all ticked upward in the latter half of April, state officials said Friday. The average number of new COVID-19 cases reported each day in Colorado has doubled over the past month, and the positivity rate has ticked above the 5% threshold, a sign that the virus is spreading faster.

But still, both of those metrics – along with hospitalizations – are far below what they were in mid-January or during much of the delta wave that swept through the state in the latter part of 2021.

The recent increase is primarily attributable to BA.2, a subvariant of omicron, which accounts for 80% of new infections in the state. It’s more transmissible – but not more severe – than omicron. 

But it’s being shadowed now by a second subvariant, BA.2.12.1, which is more transmissible than both BA.2 and the initial omicron strain. This latest version caused 11% of the new cases in the state, and Rachel Herlihy, the state’s epidemiologist, told reporters Friday that it “probably could become the dominant here in the United States and in Colorado.”

Part of what makes omicron so disruptive is waning immunity from vaccinations and its ability to evade previous immunity from infections, like delta. But the sheer scale of infections from the omicron wave, which began in December, helped end the wave within two months and, experts have said, should give Coloradans broad immunity into the early summer. 

Herlihy said BA.2.12.1 – the most recent subvariant – is still new and is being studied but that it doesn’t appear to be able to evade immunity from previous omicron infections. That’s a good sign, she said.

Cases, the positivity rate and hospitalizations are all likely to continue to increase in the coming weeks, Herlihy said. But they’re unlikely to match previous highs, according to a team of researchers who released their latest pandemic projections Wednesday.

“BA.2 could cause an increase in COVID-19 cases and hospitalizations in the next three months, but hospitalization peaks are projected to be well below prior peaks,” the researchers, led by the Colorado School of Public Health, wrote.

Herlihy told reporters that projections don’t indicate that hospitals will be strained to the degree they were just a few months ago. Hospital capacity has long been the most-watched metric of state officials, particularly given the high level of vaccination among the population.

Scott Bookman, the state’s incident commander for COVID-19, told reporters that officials were monitoring the increase and would react as necessary. The state had begun to step back from its active role in testing, vaccinating and treating COVID-19 cases, given the decline in cases, wide uptake of vaccinations and the stabilization of the health care system. 

Herlihy and other officials, including the modeling team, have said Colorado’s high rate of immunity – from vaccination and from the number of infections from omicron – should give the state some protection, even if temporary, against large waves.

That could change if a new variant, which is both highly transmissible and capable of evading prior immunity, emerged. Thus far, omicron has been followed in dominance by its own progeny. But should that family tree be replaced atop the hill by a more concerning variant, officials have said, cases and hospitalizations could jerk back to levels seen during the worst peaks of the pandemic.