Susan Hopkins, the chief medical adviser at the UK Health Security Agency, said on Thursday (December 16) that so far there were 15 proven cases of Omicron in hospitals, but that the number was likely to be much higher.
"I think the earliest that we will have reliable data is the week between Christmas and New Year and probably early January," she told a parliamentary committee.
Hopkins said experts would need about 250 individuals in hospital to make a severity assessment compared with the earlier Delta variant.
"We start running that assessment when we start having enough cases who have been admitted hospital to do it and then we run it daily," she said.
Chris Whitty, England's chief medical officer, also told the committee that the numbers of hospitalisations could rise rapidly and could see more people being admitted to hospital on a single day than during the previous peak in January.