What is driving Colorado’s COVID surge? Not even the experts are sure.

The number of people hospitalized with coronavirus infections in Colorado fell slightly Thursday, offering a bit more breathing room to the state’s overloaded hospitals.

But, as hospitals begin implementing more aggressive transfer protocols and health care workers continue to sound an alarm about the direction of the pandemic in Colorado, no one is celebrating.

“For now, the curve is going to keep going up, barring something unexpected,” said Dr. Jonathan Samet, the dean of the Colorado School of Public Health and the leader of the state’s COVID-19 Modeling Group.

But then, he added: “I’m getting wary of making predictions because we’ve seen so many unexpected twists and turns.”

Even as Colorado approaches two years in the pandemic, there is still much the state’s leading authorities on the virus do not know. That extends to the current surge in cases, which has defied expectations and conventional wisdom and placed hospitals in jeopardy of being overwhelmed, a risk many believed had long passed.

Nationwide, coronavirus infections have declined from their peaks of earlier this fall, more recently leveling out. But, in Colorado, one of the more heavily vaccinated states in the country, infections have risen. And no one is quite sure why.

“I don’t have a great answer for that and, believe me, I’m looking,” Dr. Rachel Herlihy, the state epidemiologist, said last week.

Meanwhile, hospitals are being hit not just by coronavirus hospitalizations but the consequences of people delaying health care during the pandemic.

Here are five numbers to help explain the current crisis in the state.

As recently as a few weeks ago, Colorado had been among the states with the lowest infection rates in the nation.

The state’s rise in the rankings has coincided with an extended case surge across the western half of the United States. Eight of the top 10 states in the nation for infection rates are west of the Mississippi River (including Alaska, which tops the list). Wyoming has the highest per-capita COVID death rate over the past week. Montana has the highest hospitalization rate.

“For whatever reason, our region, the mountain West and upper Midwest is experiencing a significant rise in COVID-19 right now,” Gov. Jared Polis said this week.

Earlier this year, some of the nation’s leading experts on infectious diseases had begun to talk about a curious pattern they were seeing in coronavirus case surges: They always seemed to last about two months.

Infection spikes in Colorado had followed this trend. In the state’s biggest surge, last year, cases began rising in late September and peaked by late November. Previous and subsequent increases in infections had risen for no more than two months before falling.

But this latest wave has broken the pattern. Infections in Colorado began increasing this year in early July and may still not have peaked.

“What we are having trouble identifying is why are we on this upward path?” Samet said. “That is a question I wish we could answer.”

There are a lot of factors favorable to transmission now, Samet said. People are moving about and mixing more in crowded places. Mask-wearing has dropped way off. But those factors exist in other states, too, and their cases have declined.

Herlihy said the changing seasons may have something to do with it.

“I think it’s probably more than a coincidence that this is the exact time that we saw an increase last year,” she said. “I think there is something with seasonality, human behavior. It’s hard to know. Is it temperature, is it humidity, is it human behavior?”

The majority of people in Colorado hospitals right now do not have COVID-19.

But the surge comes at a particularly difficult time for hospitals, making it tougher for them to handle a rising number of coronavirus patients.

When hospitalizations for COVID-19 peaked in Colorado last December — at more than 1,800 patients — coronavirus cases made up nearly 18% of all people hospitalized. But there were fewer people hospitalized for other things at the time. The state’s hospitals were around 80% full.

Now, hospitals are about 90% full — and intensive care units are even more so, around 93%. And there are also just fewer beds in the hospitals, largely due to staffing issues.

At times during the pandemic, the state has had the ability to fill more than 1,900 staffed ICU beds. On Thursday, the state counted 1,587 staffed ICU beds across its hospitals.

What has changed in the meantime is that health care workers — burnt out from months of caring for so many patients who were so sick, tired of abuse they have received from angry patients and families, lured away by higher-paying traveling jobs that send them to wherever need is greatest — have left their jobs.

“So the capacity has shifted,” said Dr. Anuj Mehta, a critical care pulmonologist at Denver Health. “It’s not just a function of patients. I think our capacity is less than it was a year ago.”

There’s another impact of the virus that is straining hospitals right now: For more than a year and a half, people have been putting off medical care that they needed, trying to avoid a visit to the hospital or doctor’s office during a global pandemic. At first, the care was just preventive — colonoscopies, mammograms, routine checkups.

But, as the months rolled on, the treatable things that were missed grew more and more serious. And now they are haunting Colorado hospitals.

Mehta said patients are coming in with advance-stage cancer or organ failure. He said ICUs are seeing patients coming in with complications related to HIV infections — a rarity in recent years when improvements in prescription drugs have helped patients control their infections.

“Rather than getting routine care in their doctor’s office or their provider’s office, they’re coming in now for emergency care,” he said of many patients generally.

And those patients are also taking up hospital bed space.

State hospital leaders are now trying to walk a messaging tightrope. They want Coloradans to understand how full hospitals are right now, in the hopes it will inspire people to take action to reduce their risk of coronavirus infection. Follow the three W’s, they say — wear a mask, wash your hands and watch your distance.

But they also want to be sure people know hospitals are open for those who need them. This is why hospitals are OK with Polis issuing an order suspending cosmetic procedures at hospitals but not all elective procedures, as he did at the start of the pandemic. Those elective procedures could prevent the need for emergency care down the road.

“Too many people did not get care when they heard we were closed,” said Julie Lonborg, a spokeswoman for the Colorado Hospital Association. “We do not want to make that mistake again.”

About 62% of the people living in Colorado are fully vaccinated against COVID-19. When looking just at people ages 12 and older — the group that was, until this week, considered the “vaccine-eligible population” — the rate is around 72%.

Unvaccinated people, though, make up the majority of people being infected and hospitalized with coronavirus in Colorado, both in terms of rates and in terms of raw numbers.

For the week of Oct. 17 — the most recent data available on breakthrough cases in the state — about 16,000 people were diagnosed with coronavirus infections in the state, according to numbers from the Colorado Department of Public Health and Environment. Of those, about 11,000 were unvaccinated.

Hospitalizations show an even greater disparity. CDPHE says that 80% of people currently hospitalized with COVID-19 are unvaccinated. Each week in Colorado, unvaccinated people are hospitalized at a rate roughly 4½ times greater than vaccinated people.

And that, for Colorado health care leaders, may be the most puzzling stat of all.

“I’m flabbergasted,” Mehta said, “that we’re at this point, 20 months into the pandemic, when we have truly borderline miraculous vaccines.”