Its authors claimed the variant was “behaving in a way that is less severe” following a comprehensive review of infection and hospital data in the past month.
It comes as ministers in the UK are poised to decide whether to impose new restrictions after Christmas to curb the spread of the highly transmissible new strain.
The study also found that, for those who were admitted to hospital, the risk of severe disease was roughly 30 per cent lower.
The new study, which has not been peer-reviewed, assessed the severity of disease by comparing data about Omicron infections in October and November with data about Delta infections between April and November — all in South Africa.
The analysis was carried out by scientists from the National Institute for Communicable Diseases (NICD) and major universities.
Professor Cheryl Cohen, of the NICD, said: “In South Africa, this is the epidemiology: Omicron is behaving in a way that is less severe.”
But the study noted several important caveats — including the high level of antibodies in South Africa’s population from previous waves of infection.
The authors wrote: “It is difficult to disentangle the relative contribution of high levels of previous population immunity versus intrinsic lower virulence to the observed lower disease severity.”
They make clear it is not known what would happen if Omicron spread in a population with less immunity.
Paul Hunter, a professor of medicine at the University of East Anglia, also noted the study compares Omicron data from one period with Delta data from an earlier period.
“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,” he said.
“To a certain extent this does not matter to the patient who only cares that they won’t get very sick. But it is important to know to enable improved understanding of the likely pressures on health services.”
Results of a major study from Imperial College London released last week showed there was no sign Omicron was less severe than Delta. However, data on hospitalisations in the UK remains very limited.