A preliminary US study of nearly 70,000 COVID-positive people showed a substantially reduced risk of hospitalization and death from Omicron even after controlling for growing population immunity levels.

People infected with Omicron were half as likely to be hospitalized, about 75 percent less likely to need intensive care, and around 90 percent less likely to die compared to those infected with the formerly dominant Delta variant, according to the paper.

Of some 52,000 people infected with Omicron, none ended up on a ventilator, compared to 11 people from nearly 17,000 with Delta.

Hospital stays lasted for a median of 1.5 days for Omicron compared to five days for Delta, and 90 percent of Omicron patients were discharged in three or fewer days.

The analysis was conducted on data from the Kaiser Permanente Southern California hospital system, which serves a population of around 4.7 million people, between November 30, 2021, and January 1, 2022, when both strains were circulating widely.

The findings build on accumulating population-level research from countries including South Africa and Britain, but also on animal and human tissue-based testing, which have found Omicron replicates better in the upper airways compared to the lungs.

The new paper, which has not yet been peer-reviewed, was carried out by researchers from the University of California, Berkeley, Kaiser Permanente and the Centers for Disease Control and Prevention (CDC).

"This study controlled for important key parameters such as age, sex, prior SARS-CoV-2 infection, prior vaccination and comorbidities," CDC director Rochelle Walensky told reporters on a briefing call Wednesday.

The results thus suggest that Omicron is "intrinsically less severe than Delta," and observed reductions in severe cases aren't only the result of more people being vaccinated and infected over time, the paper said.

In addition, while the study noted reduced vaccine efficacy against infection from Omicron, it also found substantial ongoing protection against severe outcomes.

Walensky warned that the results should not lead to complacency, since Omicron's extreme transmissibility is still stretching the United States' already over-extended health care system and its exhausted health workers.

The country is currently seeing an average of 750,000 cases a day -- though that figure is soon expected to exceed a million -- around 150,000 total COVID hospitalizations, and more than 1,600 daily deaths.

President Joe Biden's chief medical advisor Anthony Fauci predicted Tuesday that "Omicron, with its extraordinary, unprecedented degree of efficiency of transmissibility, will ultimately find just about everybody."

But he added that after the country emerged from its current wave, it would transition towards a future of living with the virus, with COVID vaccines moderating severe disease for the majority and effective treatments available for the most vulnerable.