Scientists caution that any reductions in severity need to be weighed against the fact that Omicron spreads much faster than Delta and is more able to evade vaccines.
The agency's research said the protection a booster shot of vaccine gives against symptomatic Omicron infection appears to wane after about 10 weeks, though protection against hospitalisation and severe disease is likely to hold up for longer.
UKHSA chief executive Jenny Harries said the analysis "shows an encouraging early signal that people who contract the Omicron variant may be at a relatively lower risk of hospitalisation than those who contract other variants".
But she added that "cases are currently very high in the U.K., and even a relatively low proportion requiring hospitalisation could result in a significant number of people becoming seriously ill".
U.K. Health Secretary Sajid Javid said the emerging information about Omicron was "encouraging news," But he said it was "not very clear yet ... by how much that risk is reduced" compared to Delta.
The analysis follows two studies, from Imperial College London and Scottish researchers, that found patients with Omicron were between 20% and 68% less likely to require hospital treatment than those with Delta.
Data out of South Africa, where the variant was first detected, have also suggested Omicron might be milder there.
Even if the early studies are borne out, the new variant could still overwhelm health systems because of the sheer number of infections. The British health agency said omicron appeared able to re-infect people more easily than previous variants, with 9.5% of Omicron cases found in people who had already had Covid-19 — a figure it said was likely an underestimate.
Countries around the world are looking closely at Britain, where Omicron is now dominant and where Covid-19 cases have surged by more than 50% in a week.
Britain reported 119,789 lab-confirmed Covid-19 cases on Thursday, the highest yet during the pandemic and the second day the number has topped 100,000.
Britain's Office for National Statistics estimated that about 1 in 45 people in private households in England — 1.2 million individuals — had Covid-19 in the week to Dec. 16, the highest level seen in the pandemic.
Britain's Conservative government this month reinstated rules requiring face masks in shops and ordered people to show proof of vaccination or a negative coronavirus test before entering nightclubs and other crowded venues in an attempt to slow omicron's spread.
The government said Thursday it would not impose any new restrictions before Christmas, but might do so soon after.
Officials also urged people to get tested regularly and cut back on socialising. Many in Britain have heeded that advice, leaving entertainment and hospitality businesses reeling at what should be their busiest time of the year.
The government has offered grants and loans to support restaurants, bars, theatres and other venues, but many say it is not enough to stop them going under.
Rules set by the U.K. government apply in England. Other parts of the U.K. — Scotland, Wales and Northern Ireland — have set slightly tighter restrictions, including the closure of nightclubs.
The government is hoping vaccine boosters will provide a bulwark against Omicron, as the data suggests, and has set a goal of offering everyone 18 and up a third shot by the end of December.
