LONDON — Omicron is “the most significant threat” to public health in the U.K., authorities warned Wednesday, as the World Health Organization cautioned that the highly contagious Covid-19 variant was spreading at an unprecedented rate. Britain reported 59,610 new Covid infections on Tuesday, the highest figure since early January and the fifth highest recorded since the outbreak of the pandemic in March last year. More than 5,300 cases of omicron have been recorded in the U.K., with 10 people hospitalized. One person died after contracting the variant. New regulations intended to help slow the spread of omicron, including masks in...