People with Omicron are significantly less likely to develop severe symptoms, according to new analysis by the UK Health Security Agency (UKHSA) released on 23 December.

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Early results suggest people are 30 to 45 per cent less likely to go to A&E if they are infected with Omicron rather than Delta. They are also 50 to 70 per cent less likely to be admitted to hospital. The findings also show that the preventive effects of a booster COVID vaccine dose wane after 10 weeks but experts stressed that vaccines remain the best defence against severe disease.

Omicron is spreading fast and the COVID-19 vaccine remains our best line of defence against it, said Dr Jenny Harries, UKHSA Chief Executive. The UKHSA analysis is based on all cases of Omicron and Delta in the UK since the beginning of November, including 132 people admitted to the hospital with the variant. There have also been 14 deaths in people within 28 days of catching Omicron. The findings coincide with two other studies based on real-world COVID-19 UK data, which also report that the Omicron variant is less severe than the Delta variant, with fewer infected people requiring hospitalisation. 

Research by Imperial College London found that people with PCR-confirmed Omicron are 40-45 per cent less likely to spend a night or more in hospital compared with Delta. Those with Omicron after a previous infection are 50-60 per cent less likely to be hospitalised, compared with those with no previous infection. However, the risk of hospitalisation is higher for those who are unvaccinated, according to the study.

"Our analysis shows evidence of a moderate reduction in the risk of hospitalisation associated with the Omicron variant compared with the Delta variant, said Imperial's Professor Neil Ferguson, who is the scientist associated with the UK's first complete stay at home lockdown during the first COVID wave in March 2020.

However, this appears to be offset by the reduced efficacy of vaccines against infection with the Omicron variant," he cautioned. The "high transmissibility" of Omicron could lead to health services facing "increasing demand" if cases grow at the rate seen recently, the scientist said after the UK recorded over 100,000 daily COVID infections on Wednesday.

The sample for the Imperial College study, which is yet to be peer-reviewed, included 56,000 Omicron cases and 269,000 Delta cases. Another research by the University of Edinburgh and other experts in Scotland, based on a small sample of 15 people in hospital, also concluded a two-thirds reduction in the risk of COVID-19 hospitalisation when compared to Delta.

Dr Jim McMenamin, the national COVID-19 incident director for Public Health Scotland, said the findings were "a qualified good news story", but that it was "important we don't get ahead of ourselves". He said a "smaller proportion of a much greater number of cases" could still mean a "substantial" number of people may experience severe COVID infections which could lead to hospitalisation.

Further data from South Africa suggests people catching COVID are 80 per cent less likely to be taken to hospital with Omicron, compared with other strains. Patients are 70 per cent less likely to develop severe disease compared with Delta, according to the National Institute for Communicable Diseases.

But worldwide researchers have stressed the severity of the variant could be different for other populations and also effects on older age groups is yet to be fully studied.

Ministers have stressed the UK government is constantly considering new COVID data, while other parts of the UK have already gone ahead with announcing post-Christmas rules of limiting numbers of households mixing. Scotland, Wales and Northern Ireland have all set out restrictions from next week that will ban nightclubs and limit large groups from gathering.