California hospitals find that Omicron causes fewer hospitalizations and shorter stays.

A nurse talking with a Covid-19 patient at Providence Holy Cross Medical Center in Los Angeles in December.
Credit...Jae C. Hong/Associated Press

A new study of nearly 70,000 Covid patients in California demonstrates that Omicron causes less severe disease than other coronavirus variants, results that align with similar findings from South Africa, Britain and Denmark, as well as a host of experiments on animals.

Compared with Delta, Omicron infections were half as likely to send people to the hospital. Out of more than 52,000 Omicron patients identified from electronic medical records of Kaiser Permanente of Southern California, a large health system, the researchers found that not a single patient went on a ventilator during that time.

“It’s truly a viral factor that accounts for reduced severity,” said Dr. Lewnard, an epidemiologist at the University of California, Berkeley who was an author of the study, which was posted online on Tuesday and has not yet been published in a scientific journal.

Despite the less severe virulence of Omicron, U.S. hospitals are buckling under an influx of coronavirus cases. Dr. Lewnard said that this was the result of the variant spreading like wildfire. On average, more than 730,000 people are testing positive every day in the United States, almost three times the previous peak last winter.

“Since it’s more transmissible, there will just at some point be a lot of hospitalizations that inevitably occur,” Dr. Lewnard said.

In recent weeks, Britain and several other countries have reported that Omicron has a lower risk of hospitalizations. When the variant hit the United States last month, Dr. Lewnard and his colleagues began analyzing electronic health care records maintained by Kaiser Permanente of Southern California, which serves 4.7 million people.

They analyzed 69,279 symptomatic patients who tested positive for the coronavirus from Nov. 30 to Jan. 1. Three-quarters of the positive samples contained the Omicron variant, and the rest were Delta.

The researchers then followed the people who tested positive to see whether they ended up in the hospital. They excluded the so-called incidental Covid patients who showed up at hospitals for other complaints and tested positive for the coronavirus after arriving.

Compared with Delta, Omicron cut the risk of hospitalization in half, the study found, and the people who came to the hospital with Omicron stayed for a shorter period. The variant cut hospital stays by more than three days, a reduction of 70 percent compared with Delta.

Fourteen of the Delta-infected patients died, while only one Omicron patient did.

As scientists have gathered evidence that Omicron is less severe, they have struggled to understand why. One reason is that the people infected with Omicron have more immune defenses than in previous waves.

In other countries, researchers have found that earlier infections with other variants reduce the chances that people end up severely ill with Omicron. Vaccination also offers protection.

“Vaccines are quite helpful,” Dr. Lewnard said. He and his colleagues found that Californians who were vaccinated were 64 to 73 percent less likely to be hospitalized than unvaccinated people.

Even among unvaccinated people, however, Omicron was less likely to lead to hospitalizations than Delta.

Dr. Lewnard said that this extra analysis showed that Omicron was fundamentally less severe. Studies on animals suggest that Omicron readily infects cells in the upper airway but works poorly in the lungs, which could explain its milder effects.