More patients are getting COVID-19 during hospital stays. Experts worry it's because infected healthcare workers are sick on the job.

• The CDC announced looser isolation rules for healthcare workers in December amid staffing shortages.
• The number of inpatients who contracted COVID-19 during their hospital stays rose shortly afterward.

The Centers for Disease Control and Prevention stunned many disease experts last month when it announced healthcare workers could return to work seven days after testing positive for COVID-19, instead of its previous 10-day recommendation.

The policy applies to people who are asymptomatic, or whose mild or moderate symptoms are improving, and test negative within 48 hours of returning to work. But the CDC said the isolation period could be cut even more — down to five days — in the event of staffing shortages. In that case, healthcare workers wouldn't need to test out of isolation. And in a crisis scenario, when there's no longer enough staff to provide safe patient care, there would be no work restrictions at all, the CDC said.

Nearly one-quarter of US hospitals are reporting critical staffing shortages, according to the latest data from the US Department of Health and Human Services (HHS). Often, that means having to choose between treating sick patients and allowing infected employees to return to work (though hospitals can decide for themselves what constitutes a critical shortage, NPR reported).

But disease experts fear the CDC policy is fueling in-hospital transmission, since research shows that some people with COVID-19 can still be infectious for up to 10 days.

"It's a little bit of pandemic theater. You're making the decision to bring healthcare workers back when they're sick," Susan Butler-Wu, an associate professor of clinical pathology at the University of Southern California, told Insider last month. "I don't think the data support that."

A week after the CDC's announcement on December 23, the total number of hospitalized patients who contracted COVID-19 at least two weeks into their hospital stay went up 80% — from around 1,200 to 2,200 patients — according to HHS data.

In-hospital COVID-19 transmission in the US

Those patients "initially came into the hospital for something other than COVID and then were found to be positive," Dr. Jorge Caballero, a data scientist with the nonprofit Coders Against COVID, told Insider. "The only place that they can possibly get COVID is in the hospital, because that's where they've been and they didn't have it to begin with."

Dr. Jeremy Faust, an emergency medicine physician at Brigham and Women's Hospital, said many hospitals are implementing the CDC's recommendations before they reach critical staffing shortages.

"If it's choice between nothing and somebody who came back to work a little sooner than they should and wears PPE, I would take the latter," he said. "But we should not do that unless it's absolutely necessary because bringing people back to work sooner does increase the risk of spread. You have to decide if that increased risk is worth it. In a lot of cases, it's not."

Omicron's transmissibility doesn't fully explain the sharp rise of COVID-19 in hospitals

In-hospital transmission is rising on the local level, too.

"When we look specifically at large hospital systems where there was a quick implementation of the new CDC policies, we see this huge jump," Caballero said.

On December 31, Rhode Island's Department of Health updated its isolation guidance to reflect the new CDC recommendations for hospital and nursing home workers. Within days, the number of inpatients who contracted COVID-19 during their hospital stay skyrocketed.

In-hospital COVID-19 transmission in Rhode Island

Of course, correlation doesn't equal causation: Omicron is more transmissible than other variants, so it could be driving some degree of in-hospital transmission on its own. The US saw a slight uptick in COVID-19 transmission within hospitals for roughly a month leading up to the new CDC rule.

But there are few explanations for the rapid spike of inpatients who contracted COVID-19, Caballero said.

"The thing that changed was the CDC's policy that shortened that isolation period," he said, adding: "If a hospital has good infection control measures, then we shouldn't see a huge rise in infections within the hospital setting."

Masks don't always prevent healthcare workers from spreading COVID-19

Insider recently spoke with four nurses — two in Kentucky, one in Florida, and another in mid-Atlantic region — who were instructed to come into work with symptomatic COVID-19. Many of them feared infecting patients, but also worried about wearing thick, tight masks for long hours when they were already congested or short of breath.

Caballero said Omicron leaves little room for mistakes in terms of mask-wearing.

"What we're learning — and we're getting more and more data by the day — is that even a fleeting exposure to Omicron is all it takes to actually get someone exposed and infected," he said. The variant is "far more efficient at getting into our cells," compared with Delta, he added.

Even healthcare workers who wear highly protective masks like an N95 may still leak infectious particles when they're coughing or sneezing, Caballero said. It's also unrealistic to assume that masks can be worn every second of a shift.

"When doctors, nurses need to eat, you take off your mask, and we know that this virus is airborne," Caballero said. "It can linger in rooms like a cloud of smoke for hours."

Faust said in-hospital transmission could primarily stem from "super-spreaders" — a few high contagious people who infect many others, even after short exposures.

"A patient could be in the hospital for a week and never leave their hospital room," he said. But if you take that patient to a radiology suite for an X-ray, and that suite has been occupied by 50 people throughout day, "if a few of them were really, really contagious, the virus could be hanging out in the air and that's where you'd get it."

No matter the scenario, experts said, the new CDC recommendation always carried the risk of more in-hospital transmission.

"We can't just continue to look the other way," Caballero said. "We need to do right by everyone involved — the patients, the patients' families, and the doctors and nurses."