Federal health officials reported Tuesday that the omicron variant caused less severe illness in hospitalized patients than earlier virus lineages, even though its explosive transmissibility has caused far more infections and led to more than 2,200 deaths a day on average, one of the highest tolls since early last year.
Despite record infections and hospitalizations caused by omicron, the percentage of hospitalized patients with severe illness is lower compared with those in earlier pandemic waves. That lower disease severity is partly the result of immune protection from higher vaccination coverage among those 5 and older, booster use and previous infection, as well as the potential lower virulence of the virus itself, according to the report. Other studies have suggested that the variant is less able to penetrate deep in the lungs.
That pattern notwithstanding, the virus spreads from person to person with frightening speed, resulting in significant numbers of hospitalizations and deaths. On Tuesday, the seven-day average of daily deaths in the United States was 2,230, the highest since late February 2021.
“People with underlying conditions, people with advanced age, people who are unvaccinated, can have a severe form of covid-19, following infection from omicron,” World Health Organization epidemiologist Maria Van Kerkhove said this week. “People are still being hospitalized with this variant, as well as dying.”
The strain on health systems stemming from the large numbers of infections underscores the importance of hospital surge capacity and the ability to adequately staff health-care systems, the report said.
The CDC report’s findings are consistent with recent studies from health systems in California and Texas, as well as from South Africa, England and Scotland. One of the studies, a preliminary report by Kaiser Permanente that has not yet been peer-reviewed, looked at nearly 70,000 covid-19 cases in Southern California from Nov. 30 to Jan. 1. It found that rates of hospitalization, admission to intensive care units, use of mechanical ventilation and death were all substantially lower in patients infected with omicron compared with the delta variant. Hospital stays were also shorter.
The CDC report looks at the ratio of cases during each period to emergency room visits and hospitalizations.
“What sets this report apart is that it focuses on the overall impact of the three waves on the health system, more than on ‘severity,’” Andrew T. Pavia, a professor of pediatrics and infectious diseases at the University of Utah, said in an email on Tuesday. He did not take part in the study.
“The omicron wave has been bad news with the largest societal and health care impact, but for infected individuals, particularly those who are vaccinated, the decreased overall severity is good news,” Pavia said.
The data reinforces the importance of being up to date with vaccinations, including booster shots. Recent data from hospitals has shown that a booster shot is critical to preventing severe outcomes during the omicron surge. In a CDC study released last week, researchers showed that a third dose of either the Pfizer-BioNTech or Moderna shots reduces the chance of hospitalization by 90 percent compared with unvaccinated people, and reduces the chance of a trip to the emergency room or urgent-care center by 82 percent.
The CDC used information from multiple surveillance systems and a large health-care database to compare disease severity during three periods of the pandemic: winter 2020-2021, the delta period (July 15 to October 2021), and the early omicron period (Dec. 19, 2021, to Jan. 15).
Vaccinations jumped from about 1.5 million people during that first period to 207 million fully inoculated and 1.6 million boosted by Jan. 10.
Investigators found the average length of hospital stay during omicron (5.5 days) was 31 percent shorter than during the winter of 2020-21, and about 27 percent shorter than during delta. The percentage of patients admitted to an ICU was also lower during the omicron wave compared with the two earlier pandemic periods, as was the percentage of patients who received ventilator support, the report found.
Among children under 18, lengths of hospital stay during omicron were similar to previous periods, but ICU admissions were lower, the report said. But the increases in children’s emergency department visits and hospitalizations during omicron may be related to lower vaccination rates compared with adults. Children under 5 are not yet eligible for the shots.
Jacqueline Dupree contributed to this report.