White House chief medical advisor Dr. Anthony Fauci on Wednesday said a growing body of data indicates that the Covid omicron variant is less severe than the delta strain, but cautioned that hospitals could still face stress through the unprecedented number of new infections sweeping across the U.S.

“Multiple sources of now-preliminary data indicate a decrease severity with omicron,” Fauci told the public during a Covid update from the White House response team. “However, we really do need more definitive assessment of severity with longer-term follow up here and in different countries.”

Fauci cited a study from Ontario, Canada, that found the risk of hospitalization or death was 65% lower among people infected with omicron compared with individuals who caught delta. The risk of admission to an intensive care unit or death from omicron was 83% lower, according to the study.

Fauci also pointed to a study from South Africa that found about 5% of infections during the omicron wave resulted in hospital admission, compared with 14% during delta. Patients admitted to the hospital during the omicron wave were 73% less likely to have severe disease compared with the delta wave, according to the data.

Fauci said the lung infection from omicron appears less severe than previous variants, citing recent animal studies of mice and hamsters.

“It was shown that the virus of omicron proliferates very well in the upper airway and bronchi, but actually very poorly in the lungs,” Fauci said. While this does not definitively prove omicron is more mild, it is consistent with the variant transmitting very quickly but causing less severe lung infections,” he said.

Read more: Omicron poses “very high” global risk, world must prepare -WHO

Fauci said the omicron variant also appears less severe for children when compared with delta. However, he cautioned that hospitalizations are rising among kids, mostly the unvaccinated, because omicron is so contagious. He urged parents of children ages 5 to 17 make sure their kids are immunized against Covid.

Fauci warned that even if omicron proves less severe, the variant is spreading so quickly that it could still produce a large number of patients who do require hospital care, straining the nation’s health-care system.

The U.S. reported more than 869,000 new Covid infections on Tuesday, according to data compiled by Johns Hopkins University. The nation is reporting a seven-day average of more than 553,000 new infections every day, more than double the previous week and a pandemic record, according to a CNBC analysis of data from Johns Hopkins.

“A certain proportion of a large volume of cases no matter what are going to be severe,” Fauci said. “So don’t take this as a signal that we can pull back from the recommendations.”

About 110,000 Americans are hospitalized with Covid, according to a seven-day average of data from the Department of Health and Human Services as of Wednesday, up 39% over the past week. Though rising sharply, that figure is still below peak levels seen during last winter’s surge, when hospitalizations topped 137,000 in early January 2021.

Fauci cautioned against complacency, urging the public to follow public health guidance by getting vaccinated, boosted and wearing a mask.