PROMIT MUKHERJEE THE heavily mutated Omicron variant of the coronavirus is rapidly becoming dominant in South Africa, less than four weeks after being detected there, authorities have said, as other countries tightened their borders against the new threat. The United States identified its first case of the new variant in a fully vaccinated traveller who had returned from South Africa on November 22 and had mild symptoms, top U.S. infectious disease expert Anthony Fauci said. Hours earlier, airlines in the United States had been told to hand over the names of passengers from parts of southern Africa hit by Omicron,…