A South African study suggests reduced risks of hospitalisation and severe disease in people infected with the Omicron coronavirus variant versus the Delta one, though the authors say some of that is likely due to high population immunity.
Questions about Omicron's virulence are at the heart of scientific and political debate in many countries, as governments grapple with how to respond to the spread of the variant while researchers race to understand it.
The new study, which has not been peer-reviewed, sought to assess the severity of disease by comparing data about Omicron infections in October and November with data about Delta infections between April and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 a group of scientists from the National Institute for Communicable Diseases (NICD) and major universities including University of the Witwatersrand and University of KwaZulu-Natal.
They used data from four sources: national Covid-19 case data reported to the NICD, public sector laboratories, one large private sector lab and genome data for clinical specimens sent to NICD from private and public diagnostic labs across the country.
The authors found the risk of hospital admission was roughly 80% lower for those infected with Omicron compared with Delta, and that for those in hospital the risk of severe disease was roughly 30% lower.
However, they included several caveats and cautioned against jumping to conclusions about the intrinsic characteristics of Omicron.
"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 wrote.
Paul Hunter, a professor of medicine at Britain's University of East Anglia, described the South African study as important and said it was the first properly conducted study to appear in pre-print form on the issue of Omicron versus Delta severity.
But Hunter said its main weakness was that it compared 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 by Imperial College London released last week showed there was no sign that Omicron was milder than Delta, although data on hospitalisations remains very limited. It has not been peer reviewed and published in a medical journal.