A smaller share of Americans with coronavirus cases have been admitted to hospitals during the surge of the Omicron variant than during previous waves, the Centers for Disease Control and Prevention said on Tuesday.

The report added to heartening signs from other countries and from certain health systems in the United States that Omicron, while highly contagious, is causing less severe illness — a result of growing levels of immunity in the population, as well as Omicron itself often triggering less serious symptoms.

Yet the report also offers some cause for concern. Because of soaring infection levels, the peak level of daily hospital admissions was higher than during previous surges. And deaths in the United States have risen in recent days.

The number of people infected with the coronavirus visiting emergency departments was 86 percent higher than during the Delta wave in the fall, the C.D.C. said, and the number of people with Covid admitted to the hospital was 76 percent higher.

The agency tracked data up until Jan. 15, when it said that the increase in hospital admissions appeared to be slowing. Up until that point, the average number of Americans dying from Covid every day was lower than during previous waves. But the death toll has since been climbing, and in recent days surpassed the peak average number of daily deaths during the Delta-driven surge of cases in the fall.

The C.D.C. attributed the tendency of Omicron cases to cause less severe illness to the virus itself, as well as growing levels of immunity from prior infections and from the rollout of vaccinations. It said that roughly 30 million more people were fully vaccinated during the Omicron surge than during the Delta wave in the fall.

As a result, a smaller proportion of coronavirus cases are ending in hospitalizations: The C.D.C. said there had been a peak of 27 hospital admissions for every 1,000 cases in January, compared with 78 admissions per 1,000 cases in the fall.

And the share of hospitalized Covid patients admitted to intensive care units during the Omicron wave was 29 percent lower than last winter and 26 percent lower than in the fall, the C.D.C. said.

The agency cautioned, though, that a number of factors made it tricky to compare the toll from different waves. Its hospital data, for example, did not rule out people whose coronavirus infections had nothing to do with their hospitalization.

And the period that the C.D.C. referred to as the Omicron surge, starting on Dec. 19, overlapped with the period when people infected with the Delta variant would still have been in hospitals, making it more difficult to distinguish the impact of one variant from another.