Omicron infection less likely to cause serious illness than Delta, South Africa data suggests

People infected with the Omicron variant are less likely to fall seriously ill with Covid compared to Delta, according to the first real-world study from South Africa.

Early data from the UK Health Security Agency (UKHSA) also reportedly indicates the new variant typically leads to milder symptoms.

However, the scientists behind the South African study warned immunity from infection or vaccination is likely to explain some of the reduction in risk.

The new study, which has not been peer-reviewed, sought to assess the severity of disease by comparing data on Omicron infections in October and November with data on Delta infections between April and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.

The risk of hospital admission was roughly 80 per cent lower for those infected with Omicron compared with Delta, while for those in hospital the risk of developing severe disease was roughly 30 per cent lower.

However, the authors 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.

South Africa has experienced a number of large Covid waves throughout the pandemic, leading to the mass infection of millions. The study estimates that between 60 to 70 per cent of the population had been previously infected by Covid by the end of the Delta wave.

It’s unclear how the findings will translate to the UK, which is further along in its vaccination rollout and has experienced different levels of infection among an older population more vulnerable to waning immunity.

However, early data from the UK Health Security Agency (UKHSA) appears to echo conclusions drawn by the South African study.

According to Politico, the UKHSA has found that infections of Omicron are more mild than Delta, though the agency believes this drop in virulence won’t be significant enough to avoid a large number of hospital admissions across Britain.

UKHSA has also found that there is still a high chance of hospitalisation and death among those who fall ill with Omicron, the report claims.

The data is set to be released in the UKHSA’s next variant technical briefing, due to be published on Thursday.

Paul Hunter, a professor of medicine at the University of East Anglia, described the South African study as important and said it was the first of its kind to detail the differences in severity between Omicron and Delta infections.

But Prof 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 non-peer reviewed study by Imperial College London, released last week, showed there was no sign that Omicron was milder than Delta, although it acknowledged that its data on hospital admissions was very limited.