Early hospital data from South Africa shows less than a third of patients admitted for COVID-19 during the latest wave linked to the Omicron variant are suffering severe illness, compared with two thirds in the early stages of the last two waves.

Data released by the National Institute for Communicable Diseases (NICD) for Tshwane, the metropolitan area which includes Pretoria where the first suspected outbreak of the Omicron occurred, showed 1,633 admissions in public and private hospitals for COVID-19 between Nov. 14 and Dec. 8.

Of those, 31% were severe cases - defined as patients needing oxygen or mechanical ventilation - compared with 66% early in the second wave of the coronavirus pandemic and 67% in the early weeks of the first.

Later in the day, the NICD produced data showing 22,391 new cases were confirmed on Thursday - a record so far in this fourth wave of infections - though only 22 deaths.

South African scientists first sounded the alarm on the Omicron variant late last month, when they noticed it had an unusually large number of mutatations, especially on the spike protein that the virus uses to enter human cells.

Since then, they have been urgently trying to figure out whether the mutations make Omicron more transmissible or more severe, and the extent to which they can help it blunt protection from vaccines or past COVID-19 illness.

The NICD cautioned that the study had some inherent limitations - it is not yet peer-reviewed - and that severe cases could rise as the fourth wave gets going.

"It may take a few weeks for hospitalisation outcomes to accumulate," the report said.

The report also said nothing about whether the patients studied had been vaccinated, so it wasn't clear to what extent higher vaccine coverage was keeping symptoms milder.

Early evidence suggests Omicron is far more transmissible than any previous variant, but that symptoms may be less severe, with lower levels of hospitalisation, especially in vaccinated patients.