Colorado officially surpassed 12,000 COVID-19 deaths Monday, and state officials expect the toll to continue to increase in the coming weeks, even as the omicron variant subsides and the state enters a period of pandemic calm.

The latest grim milestone arrives 10 months after the vaccine became available to every adult in the state, a vaccine that has proven to significantly reduce mortality. Roughly a third of Colorado’s 12,112 deaths due to the virus have been confirmed since October, the bulk of them unvaccinated. Those recent fatalities came amid the most prolonged surges of the pandemic, driven first by the delta variant and then omicron. While that latter strain has proven to be less severe than its predecessor, it has not shielded many Coloradans from becoming part of an ever-growing death toll.

Across the United States, deaths have spiked. The nation surpassed 900,000 deaths due to COVID-19 earlier this month, greater than the number of people who live in Indianapolis or San Francisco. The nation has regularly averaged more than 2,000 deaths per day of late, spurred on by an omicron wave that sent case and hospitalization rates skyrocketing over the past two months.

“It is an astronomically high number,” Ashish K. Jha, the dean of the Brown University School of Public Health, told The Associated Press earlier this month. “If you had told most Americans two years ago as this pandemic was getting going that 900,000 Americans would die over the next few years, I think most people would not have believed it.”

In Colorado, the omicron wave has largely subsided, with cases falling to the lowest levels since the surge began and one of the lowest rates since early September. Hospitalizations – which set a new high just a month ago – are now at their lowest point since late August. Gov. Jared Polis and other state leaders have said they track hospitalizations closer than any other metric, as they represent the greatest threat to Colorado society.

But deaths, a lagging indicator, have not subsided. Fatalities follow new cases by about three weeks, on average, meaning COVID-19 deaths in Colorado caused by the omicron variant would be expected to begin accumulating around the first and second week of January. Since then, the number of COVID-19 deaths has risen, hitting highs not seen since the deadly, pre-vaccine COVID-19 surge last winter.

Omicron has been responsible for most fatalities since the end of December, Rachel Herlihy, Colorado’s epidemiologist, said in an email Monday. Just over 950 deaths due to COVID-19 have been reported since the day after Christmas.

“(N)early all of those deaths are due to (the) omicron variant,” she said, “and because death data lag, we unfortunately expect to see this number increase in the coming weeks.”

Lisa Miller, an epidemiologist and associate dean at the Colorado School of Public Health, also attributed the recent rise in mortality to omicron. But she noted that it’s difficult to gauge mortality in real time: Unlike cases or hospitalizations, deaths are confirmed and processed through the Centers for Disease Control and Prevention. That process inserts a gap between when a person actually dies of the virus and when their mortality has been added to Colorado’s official death toll. That means the state likely surpassed 12,000 fatalities before Monday, when the figure became official.

Miller said it was “too early to make a lot of conclusions” about omicron and mortality because data will continue to be backfilled. 

Herlihy called the state surpassing 12,000 deaths “a sad and stark reminder that we need everyone to get vaccinated, stay up to date on recommended doses and practice public health protocols.”

Speaking at a news conference last week, Herlihy said the unvaccinated have been more likely to die than their inoculated peers. Recent state data indicates that Coloradans who are fully vaccinated and boosted are 59 times less likely to die of COVID-19 than the roughly 20% of the state that remains unvaccinated.

Miller said it wasn’t surprising that deaths rose amid omicron. 

“Unfortunately, we expected deaths to rise because we know that omicron, even though it was somewhat less severe – it still was a deadly infection for some people,” she said. “So yeah, we expected that mortality would rise, or that number of deaths would rise. And we also know that was particularly the case for unvaccinated people. … I think it’s incredibly unfortunate, and we could’ve saved lives if we’d vaccinated more people and if more people had chosen to get vaccinated and boosted.”

COVID-19 surges do not just increase mortality for those hospitalized with the disease. Between March 2020 and June 2021, for instance, overall mortality in Colorado hospitals jumped 35%, according to a recent report published by the Colorado Hospital Association. Federal researchers found that increased ICU hospitalizations from COVID lead to higher overall mortality; those increases, the study found, “fell more heavily on working-aged adults from marginalized communities who experience poor access to health care outside pandemic conditions.”

Unsurprisingly, data from the CDC shows that mortality rates – not just raw numbers – increase during surges, when more people are hospitalized with COVID-19. The highest rates, according to that data, have been in the first wave, in spring 2020; in the late 2020 wave; and in the delta and omicron surges.

It’s impossible to predict the future with COVID-19, as the past two years have demonstrated. New variants may emerge that can escape immunity, Miller said, or they may not; new antivirals should help improve mortality rates, which have already been given a boost by health care providers’ increased knowledge about how to best treat patients hospitalized with the disease.

But “unless something changes” among the unvaccinated, Miller continued, “I do think we’ll continue to see death due to COVID, especially among the unvaccinated.”

Herlihy echoed that sentiment.

“As more Coloradans become vaccinated and new cases continue to decline,” she said Monday, “we hope that serious illness and death related to COVID become less frequent. We are cautiously optimistic, but disease transmission is likely to continue at some level for the foreseeable future.”