The majority COVID-19 infections caused by the highly contagious omicron variant were reinfections, according to a study from the United Kingdom released Wednesday. Participants in the study who tested positive reported to researchers from Imperial College London whether they had tested positive in the past, and more than two-thirds said they had. The study, which analyzed 100,000 PCR tests taken in January, flies in the face of previous ideas about immunity bestowed by a prior COVID-19 infection. Scientists estimate the risk of reinfection by omicron is 5.4 times higher than with COVID-19’s once-dominant variant, delta. “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 wrote. The variant makes up for more than 90 percent of new infections in the U.S. as well, having supplanted the delta variant.