(CNN) During a period when the Omicron coronavirus variant was dominant, severe outcomes for patients hospitalized with Covid-19 -- including ICU admission, ventilator use and death -- were less common than during Delta or the first winter surge, according to a study published Tuesday by the US Centers for Disease Control and Prevention.
Also, although the daily average of new cases and hospital admissions reached record highs during the Omicron period, there were fewer hospitalizations and fewer deaths for every 1,000 cases than during the Delta period or the first winter surge. These ratios compared the peaks of each metric within each period analyzed.
Severity of disease appears to be lower during the Omicron period due to a combination of factors, according to the researchers: higher vaccination coverage including boosters, immunity acquired from previous infection and potentially lower virulence of the Omicron variant.
Among patients hospitalized with Covid-19, about 13% were admitted to the ICU during the Omicron period, compared with about 18% during Delta and the first winter surge. About 7% of patients died in the hospital during Omicron, compared with more than 12% during earlier peaks.
Also, patients hospitalized with Covid-19 during the period of Omicron predominance had a lower average length of stay than during other periods of high transmission: about 5.5 days, compared with 7.6 days during the Delta surge and about eight days during the first winter surge.
For this study, researchers analyzed Covid-19 indicators for three periods of high transmission: the first winter surge from December 2020 through February 2021, a period of Delta predominance from mid-July through October 2021 and an early period of Omicron predominance from mid-December 2021 through mid-January 2022.
"Although disease severity appears lower with the Omicron variant, the high volume of hospitalizations can strain local health care systems and the average daily number of deaths remains substantial," the researchers wrote. "This underscores the importance of national emergency preparedness, specifically, hospital surge capacity and the ability to adequately staff local health care systems."
It also reinforces the value of vaccination to prevent severe illness and death, they said.
These findings are reflective of broad trends and are not indicators of individual risk. Covid-19 hospitalization data includes incidental Covid-19 infections, which could affect in-hospital severity indicators. The Delta variant was still circulating during the beginning of the Omicron period, which could also affect severity indicators.
Researchers analyzed trends in Covid-19 cases and deaths reported to the CDC by state and territorial health departments, and in-hospital outcomes among more than 300,000 Covid-19 patients in 199 hospitals.