Cases of the Omicron variant could be spreading even faster in England than in South Africa, according to a senior scientific adviser, who warned that the variant was a “very severe setback” to hopes of bringing the pandemic under control. Prof John Edmunds, an epidemiologist at the London School of Hygiene and Tropical Medicine and a member of the government’s Scientific Advisory Group for Emergencies (Sage), said that plan B measures announced by the prime minister were “absolutely not an overreaction” even if Omicron turned out to be milder than the current dominant variant. Edmunds told a Royal Society of Medicine webinar on Thursday that it was “extremely likely” there were many more cases of Omicron in the community than those confirmed by testing, and that the numbers were set to soar in the weeks ahead. The UK Health Security Agency identified a further 249 Omicron cases on Thursday, almost twice the number announced the day before, bringing the UK total to 817. Edmunds said that if the UK had 1,000 cases today, then a doubling time of two to three days would drive the number up to 8,000 in a week and 64,000 in two weeks. Those would come on top of the continuing wave of Delta infections. “Nobody wants to have to reintroduce these measures. It’s very damaging for parts of the economy – the hospitality and retail sector, in particular, are going to be affected – but unfortunately we have to do it,” he said. “With the speed of spread of this virus, we may well have really significant numbers of cases by Christmas,” he added. “I suspect that whatever we do now, we are unlikely to overreact.” Given the rate of community transmission, he said, travel restrictions were “not really going to do much now”. The warning came as the government reported a further 50,867 daily Covid cases, 813 more admissions to hospital and 148 additional deaths on Thursday, marking rises on all measures over the past week. Speaking at the online event, Edmunds dismissed suggestions that the Omicron variant might be “good news” if it hospitalised people at only half the rate as the Delta variant. The hope comes largely from hospital admissions in South Africa, where the population is far younger than in the UK and so less likely to experience severe Covid illness in the first place. While the average age in the UK is just over 40, it is less than 28 in South Africa. Edmunds said there was “not a shred of evidence” that Omicron was half as pathogenic as the Delta variant, but added that even if this were the case, it might not make the difference people hoped for. “This is a very severe setback. There is no question about that.” “If you are worried about the time [at] which the NHS might start to get very stressed, then halving the hospitalisation rate means that buys you two to three days. I think it’s really silly to suggest this is good news; it couldn’t be further from that,” he added. “This is as bad news as you can possibly get, quite frankly.” It will take several weeks for scientists to get a sense of how severe the disease is in the UK, where the vast majority of people have some immunity against earlier variants either through vaccination or previous infection. This week, Pfizer released data from a small lab study suggesting that three shots of its vaccine might hold up well against the Omicron variant, but Edmunds said it was the most optimistic of five or so studies he had seen on vaccine protection against the variant. Despite the uncertainties, Edmunds said it was “absolutely critical” for people to get their booster doses “as rapidly as possible”: “I think it will help, and it will help very significantly, but I still think we are likely to be under quite a lot of pressure in a very short time.” Prof Peter Openshaw, a member of the government’s New and Emerging Respiratory Virus Threats Advisory Group, who also took part in the webinar, said he favoured vaccinating children aged five to 11, adding that uncertainties over the medium- to long-term consequences of Covid were “much more worrying” than the potential side-effects of the vaccine. Edmunds agreed, adding that if the Medicines and Healthcare products Regulatory Agency approved the jab for those aged five to 11, it should be rolled out as rapidly as possible. “We’re certainly not out of the woods,” Edmunds said. “I think over the next two months we’re going to see a really rather large wave of Omicron. We are going to get a large number of cases, and it will result in a large numbers of hospitalisations, and unfortunately it will result in a large number of deaths. I’m pretty sure of that.”