Researchers from Imperial College London stressed that these estimated reductions in severity must be balanced against the larger risk of infection with Omicron, due to the reduction in protection provided by both vaccination and natural infection.
For example, at a population level, large numbers of infections could still lead to large numbers of hospitalisations.
The new report estimates hospitalisation risk for Omicron cases in England. It includes all PCR-confirmed SARS-CoV-2 cases in England between December 1 and 14, where the variant causing infection could be identified from genetic data or via S gene target failure (SGTF). The data set included 56,000 cases of Omicron and 269,000 cases of Delta.
The estimates suggest that Omicron cases have, on average, a 15-20 per cent reduced risk of any hospitalisation and an approximately 40-45 per cent reduced risk of a hospitalisation resulting in a stay of one or more nights.
"Our analysis shows evidence of a moderate reduction in the risk of hospitalisation associated with the Omicron variant compared with the Delta variant. However, this appears to be offset by the reduced efficacy of vaccines against infection with the Omicron variant," said Professor Neil Ferguson from Imperial College London, in a statement.
"Given the high transmissibility of the Omicron virus, there remains the potential for health services to face increasing demand if Omicron cases continue to grow at the rate that has been seen in recent weeks," he added.
The estimates also suggest that individuals who have received at least two doses of either AstraZeneca, Pfizer or Moderna vaccines have substantially reduced risk of hospitalisation compared with primary infections with Delta in unvaccinated individuals, even if protection against infection has been largely lost against the Omicron variant.
Further, reinfection is associated with approximately a 50-60 per cent reduction in hospitalisation risk compared with primary infections.
However, after adjusting estimates to account for only one in three reinfections being identified, the estimated difference in intrinsic hospitalisation risk (that is, defined for primary infections in unvaccinated people) between Delta and Omicron is reduced (that is, 0-30 per cent reduced risk of any hospitalisation) but the estimated reduction in risk of hospitalisation due to previous infection is increased (around 55-70 per cent reduction).
The researchers said it is essential to place the severity of Omicron in the context of reinfection risk in countries, like England and South Africa, where a large proportion of the population may have already been infected.
"Whilst the reduced risk of hospitalisation with the Omicron variant is reassuring, the risk of infection remains extremely high. With the addition of the booster dose, vaccines continue to offer the best protection against infection and hospitalisation," said Professor Azra Ghani from the varsity.