A major new British study has provided encouraging data that people infected with the Omicron variant are 20% less like to visit a hospital and 40% less likely to be admitted overnight than those with the Delta variant.

The Imperial College study, published Thursday, looked at everyone with a confirmed infection of either variant in the UK between 1-14 December – for a total of more than 324,000 cases.   

At the beginning of the tracking the 164 Omicron cases detected comprised only .6% of total daily cases recorded  By day 14, however, there were 15,804 Omicron cases –  more than 50% of the nearly 30,000 cases a day being reported – reflecting the steep rise in infections from the new variant.  

“Overall, we find evidence of a reduction in the risk of hospitalisation for Omicron relative to Delta infections, averaging over all cases in the study period,” concludes the study, undertaken by researchers at Imperial College. 

“The extent of reduction is sensitive to the inclusion criteria used for cases and hospitalisation, being in the range 20-25% when using any attendance at hospital as the endpoint, and 40-45% when using hospitalisation lasting 1 day or longer or hospitalisations with the ECDS discharge field recorded as “admitted” as the endpoint.

However, the proportionately smaller rate of hospitalizations must be balanced against the “the larger risk of infection with Omicron, due to the reduction in protection provided by both vaccination and natural infection,” the researchers warned.  

Hospitalization risks for people vaccinated and previously infected

The study also found that previous COVID infection reduced the risk of any hospitalisation by approximately 50% and the risk of a hospital stay of 1+ days by 61%.

Cases vaccinated with Pfizer or Moderna for doses 1 and 2 have a similar or higher risk of hospitalisation with Omicron compared with Delta, while cases vaccinated with AstraZeneca for their primary series tended to have a lower risk of hospitalisation relative to Delta. 

The studies authors noted that the small overall numbers of people hospitalized with Omicron so far limits their ability to interpret the data – in light of vaccination status. 

“As more data accumulate, with longer periods of follow-up, assessment of more severe outcomes will become feasible,” report the study’s authors, who are part of the Imperial College COVID response team, and also a WHO Collaborating Center. 

“Currently, vaccination-status stratified hazard ratio estimates … remain very sensitive to small numbers of hospitalisation, particularly in the unvaccinated Omicron group. 

“It is quite possible that larger reductions in hospitalisation risk for Omicron vs Delta may be estimated [among  those vaccinated] for the endpoints 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 endpoints.”

Image Credits: PAHO.org, Imperial College, UK.