A highly infectious variant of COVID-19 that has spread around the world since it was first discovered in Britain late last year is between 30% and 100% more deadly than previous strains, researchers said on Wednesday. In a study that compared death rates among people in Britain infected with the new SARS-CoV-2 variant, known as B.1.1.7, against those infected with other strains, scientists said the new variant had “significantly higher” mortality.