Members of the public queue for Covid-19 vaccinations and booster jabs at St Thomas’ Hospital on December 14, 2021 in London, England.Dan Kitwood | Getty Images News | Getty ImagesOmicron has almost completely replaced the previously dominant delta variant of Covid-19 in England, a new study has found, with the vast majority of cases being possible reinfections.Imperial College London’s REACT study — which has been updated monthly since May 2020 and is funded by the U.K. government — analyzed 100,607 PCR test results from across England. Swabs were collected between Jan. 5 and Jan. 20.The study found that 99% of sequenced positive swabs came from people infected with the omicron variant, with only 1% of infections being caused by the delta variant.”We observed unprecedented levels of infection with SARS-CoV-2 in England in January 2022 and almost complete replacement of delta by omicron,” the study’s authors said in their paper released Wednesday.Reinfection capabilityTwo-thirds of the 3,582 participants who tested positive in January reported that they had already tested positive for Covid in the past. A further 7.5% of infected participants said they suspected they had previously had the virus, but had not had this confirmed with a test.When previous variants of Covid were circulating among populations, it was thought that a prior Covid infection, in which people had caught the virus and recovered, could offer some protection from reinfection. But there is concern that this is not the case with omicron, with U.K. health officials estimating in December that the risk of reinfection with omicron is 5.4 times greater than it is with the delta variant.Researchers noted on Wednesday that omicron’s infectiousness had now led to the highest Covid prevalence ever observed in the REACT study.”Vaccination (including the booster campaign) remains the mainstay of the defense against [Covid-19] given the high levels of protection against hospitalizations,” the research team said.”However, further measures beyond vaccination may be required if the very high rates of omicron infection persist, despite omicron appearing to be intrinsically less likely to cause severe disease.”Omicron is a heavily mutated, highly infectious variant of Covid that has been identified by health authorities in at least 171 countries, and which has led to record numbers of Covid cases in many countries.On Monday, WHO Director-General Tedros Adhanom Ghebreyesus told the organization’s executive board that since the variant was identified nine weeks ago, more than 80 million Covid cases had been reported to the WHO — more than were reported in the whole of 2020. Last week, an average of 100 cases were reported every three seconds, Tedros said, and someone lost their life to the virus every 12 seconds.The latest REACT study found that although coronavirus infections declined in England in early January, they then “plateaued at a high level,” with one in 23 people infected.Of the tests taken in the study’s latest round, 4.4% were positive, marking a threefold increase from December.On Tuesday, the U.K. provisionally recorded 94,326 new Covid cases and 439 deaths within 28 days of a positive test result. In late December, Covid infections peaked in the country when it recorded 246,415 positive test results in one day.England is set to scrap most of its few remaining Covid measures on Thursday, when mask wearing will no longer be mandated and the public will no longer be advised to work from home. Almost 85% of the country’s eligible population — those over the age of 12 — is fully vaccinated against Covid, while 64% have received a booster shot.BA.2 ‘variant under investigation’The Imperial College study also found that 0.4% of positive tests were caused by the BA.2 omicron sub-lineage.Omicron infections are currently comprised of four sub-variants, according to the WHO, and while the BA.1 lineage is the most dominant, trends in India, South Africa and Denmark suggest that BA.2 is becoming more prevalent.Last week, the U.K.’s Health Security Agency designated BA.2 a “variant under investigation,” Sky News reported, despite there still being few cases of the sub-lineage.