Patients at a large health system in Southern California who had the omicron Covid variant were much less likely to need hospitalization, intensive care or die from the virus compared with people infected with the delta strain, a study found this week.

Infectious disease experts found omicron patients at Kaiser Permanente Southern California were 74% less likely to end up in ICUs and 91% less likely die compared with delta patients. None of the patients with omicron required mechanical ventilation, the study found.

The risk of hospitalization was 52% lower in omicron patients compared with delta, according to the study, which has not yet been peer reviewed. Researchers are publishing studies before they are reviewed by other experts due to the urgency of the pandemic.

Hospital stays for patients with omicron were also about 3 days shorter compared with people with delta. Unvaccinated patients were also less likely to develop severe disease, according to the data.

“Reductions in disease severity associated with Omicron variant infections were evident among both vaccinated and unvaccinated patients, and among those with or without documented prior SARS-CoV-2 infection,” the team of researchers found.

Kaiser Permanente Southern California provides care to more than 4.7 million people. The study analyzed more than 52,000 omicron cases and nearly 17,000 delta cases.

The large U.S. study adds to a growing body of real-world data from the United Kingdom and South Africa indicating that the omicron variant, while more contagious, doesn’t make people as sick as the delta variant.

However, officials at the World Health Organization emphasized that omicron, although generally less severe than delta, still poses a threat to the lives of the unvaccinated, the elderly and people with underlying health conditions.

“We can definitely say that an omicron variant causes, on average, a less severe disease in any human being — but that’s on average,” said Dr. Mike Ryan, head of the WHO’s health emergencies program, during a Q&A livestreamed Tuesday on the WHO’s social media channels.

“There are hundreds of thousands of people around the world in hospital as we speak with the omicron variant, and for them that’s a very severe disease,” Ryan said. He warned that omicron still poses a “massive threat” to the lives and health of the unvaccinated, encouraging them to get the shots so they have protection as the variant rapidly spreads.

Maria Van Kerkhove, the WHO’s Covid-19 technical lead, said a lower proportion of people are dying from omicron, but the variant still poses a serious risk to the elderly and those with underlying conditions.

“We do know that mortality increases with omicron with increasing age,” Van Kerkhove said Tuesday. “We also have data from some countries that show that people with at least one underlying condition are at an increased risk of hospitalization and death, even if you have omicron as compared to delta.”

Dr. Rochelle Walensky, director for the Centers of Disease Control and Prevention, said the U.S. is reporting about 1,600 Covid deaths per day on average, a 40% increase over the previous week. However, Walensky told reporters during a White House Covid briefing that those deaths are likely due to the delta variant, because the reporting of new fatalities generally lags new infections.

The U.S. reported a pandemic record of almost 1.5 million new Covid infections on Monday with an average of about 750,000 new infections every day over the last week, according to CNBC analysis of data compiled by Johns Hopkins University. That compares with a seven-day average of about 252,000 new cases a day a year ago.

Hospitalizations are also higher than last winter’s peak — before the widespread distribution of vaccines — and continue to rise. More than 152,000 people in the U.S. were hospitalized with Covid as of Wednesday, up 18% over the last week, according to data tracked by the Department of Health and Human Services.