Britons have been hit by a pre-Christmas warning about the potential for the Omicron variant to spread rapidly through the older population at family gatherings.
Dr Jenny Harries, chief executive of UK Health Security Agency, said new data suggesting Omicron has a lower hospital admission rate than Delta was compiled when the variant was predominantly affecting younger people.
She said elderly men and women are at risk of catching the disease over the festive period which could drive hospital admission and ventilator use skywards.
“I don’t think we do know yet that this is going to be a significantly less serious disease for the population, the older population, that we’re normally most concerned about in relation to serious disease and death,” Dr Harries told BBC Radio 4’s Today programme.
“We’re not seeing very significant rises in intensive care unit utilisation or use of ventilation beds. Now that may be because a lot of the people who’ve been infected to date are actually younger people.”
On Thursday, the latest analysis by the UKHSA suggested that Omicron is milder than past Covid variants such as Delta, with between 50 and 70 per cent of people less likely to be admitted to hospital.
The agency emphasised that its findings regarding the severity of Omicron were “preliminary and highly uncertain” because of the small numbers of confirmed cases currently in hospital.
The data suggests someone with Omicron is estimated to be between 31 and 45 per cent less likely to seek emergency hospital treatment, compared with Delta, and between 50 and 70 per cent less likely to be admitted to hospital.
However, Omicron is believed to be infecting more people who have previously had Covid, with 9.5 per cent of people with Omicron having contracted it before.
Dr Harries said the findings offered a “glimmer of Christmas hope” but cautioned that the UK had not yet reached the stage where the official threat from Omicron could be downgraded.
After Omicron was first detected in the UK in November, the variant began to spread rapidly through community transmission.
Dr Harries said it had only recently started to infect middle-aged and older people, a factor which could see the data on Omicron undergo significant changes in the coming weeks.
“It’s the dominant strain now across the UK and our concerns about immunovasion have actually been recognised.
“What we’ve got now is a really fine balance between something that looks like a lower risk of hospitalisation which is great news but equally a highly transmissible variant and one that we know evades some of our immune defences,” she said.
She said the variant is doubling about every two days “in most regions” of the UK, but said the country was “still in this uncertain period” due to the many cases which go undetected and the newness of the variant.
The health agency's analysis came as the UK experienced yet another record-breaking number of daily reported Covid cases, with 119,789 reported on Thursday.
This was the second day, after Wednesday, since the pandemic began that daily lab-confirmed case rates were above 100,000.
Vaccination is also believed to give less protection against Omicron, although a booster jab provides more protection against symptomatic disease compared with the first two doses alone.
Data suggests protection starts to wane 10 weeks after booster vaccination.
Asked whether people would need further boosters so soon after having their third shot of a vaccine, Dr Harries declined to offer a time frame for doses but said it was “entirely natural” for the effectiveness of jabs to wane over time.
The agency has said that Omicron's faster rate of transmission than Delta may mean that a large number of people are likely to require hospital admission, leading to a large amount of pressure on the National Health Service.
Health Secretary Sajid Javid described the UKHSA findings as “promising”, but also said: “Hospital admissions are increasing, and we cannot risk the NHS being overwhelmed.
“This is early-stage analysis and we continue to monitor the data hour by hour.
“It is still too early to determine next steps.”
This week Britain’s Prime Minster Boris Johnson held out against pressure from scientists and decided not to impose tighter Covid restrictions before Christmas.
Instead, the government urged people to exercise caution when socialising and gathering for family celebrations.
The devolved administrations in Scotland, Wales and Northern Ireland adopted a different approach.
From Sunday, outdoor events will be limited to 50, the Welsh government has announced, and a two metre social distancing rule will apply in most public settings.
Restaurants and bars can only offer table service and people can meet in groups of a maximum of six.
In Scotland, attendance at large public events is set to be limited from Boxing Day which will make football matches spectator-free.
From Monday, hospitality venues will have to return to a previous table-service only requirement.
Nightclubs in Northern Ireland will close from December 26 and the following day will see a table-service only policy ushered in for bars and restaurants. The “Rule of Six” will be brought back, limiting the number of people that can socialise together.