Once admitted to the hospital, the risk of severe disease doesn’t differ from other variants, the authors led by scientists Nicole Walter and Cheryl Cohen said.

Compared to Delta infections in South Africa between April and November, Omicron infections are associated with a 70% lower risk of severe disease, they said. The Omicron data was collected for the two months through November.

Since being identified by South African scientists on Nov. 25, the Omicron variant has fueled record case numbers across the country. Africa’s most developed economy has fully inoculated about 44% of its adult population over a seven-month period.

In the data that are being submitted to a preprint medical publication—MedRxiv—the authors adjusted for various confounding factors that could influence the results, including age, gender and whether the cases were known reinfections. For severity of disease after admission, they also adjusted for the presence of other illnesses and prior immunization.

The study also showed that those with Omicron may have higher viral loads.

The study is “important,” though its use of so-called historic controls when comparing to the Delta infections between April and November means its outcome may be biased by time issues, said Paul Hunter, a professor of medicine at the U.K.’s University of East Anglia.

“So even though cases of Omicron were less likely to end up in hospital than cases of Delta, it is not possible to say whether this is due to inherent differences in virulence or whether this is due to higher population immunity in November compared to earlier in the year,” Hunter said. 

The authors flagged the same limitations.