New UK research on Omicron suggests those infected with the variant are less likely to be hospitalised than with Delta, and unlikely to be admitted for as long.
Scientists at Imperial College London (ICL), who published the data on Wednesday, have warned their estimated findings must be taken in the context of concerns over greater infection rates from the new variant.
However, the study will be welcomed by public health doctors trying to understand just how harmful Omicron might be in patients.
The estimates suggest that those infected are 15 per cent less likely to attend hospital and 40 per cent less likely to be hospitalised for a night or more, compared to Delta.
The authors at ICL’s Covid-19 response team believe their work will assist in refining mathematical models of potential healthcare demand associated with the unfolding European wave. The British government has been awaiting the data to help guide its own response.
“Researchers stress 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,” ICL said in a statement on Wednesday.
“For example, at a population level, large numbers of infections could still lead to large numbers of hospitalisations.”
The data set used included 56,000 cases of Omicron and 269,000 cases of Delta confirmed in England the first half of December. However, the teams said insufficient time has passed for enough data to examine the severity of more severe outcomes such as intensive care unit admission or death.
“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,” the college said.
“Reinfection is associated with approximately a 50-60 per cent reduction in hospitalisation risk compared with primary infections.”
As more data accumulates, with longer periods of follow-up, the researchers believe assessment of more severe outcomes will become feasible.
They say it is “quite possible” to estimate larger reductions in hospitalisation risk for Omicron in terms of ICU admission and death given that remaining immune protection against more severe outcomes of infection are expected to be much higher than those against milder disease.
The research also provides some insight into vaccination status. While caution is advised given the limited sample sizes, estimates suggest those with 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.
The authors also point out that the severity of Omicron must be set in the context of countries like England and South Africa where a large proportion of the population might have already been infected. Hospitalisation rates in England for Omicron are being strongly affected by infection-induced immunity.
"Our analysis shows evidence of a moderate reduction in the risk of hospitalisation associated with the Omicron variant compared with the Delta variant," said ICL's Professor Neil Ferguson.
“However, this appears to be offset by the reduced efficacy of vaccines against infection with the Omicron variant. 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.”
The findings, not yet peer-reviewed, are presented in the latest report from the World Health Organisation’s Collaborating Centre for Infectious Disease Modelling.