US COVID-19 cases surpassed 18 million on Tuesday as health officials tried to tamper fears about a new, highly transmissible variant of the coronavirus in the United Kingdom.Reports of the new virus variant in England, which prompted a pre-Christmas lockdown and caused dozens of countries to close their borders to British travelers this week, have spurred talks among government officials of mandatory COVID-19 testing for travelers from the UK and a possible quarantine mandate.News of the coronavirus mutation comes as the United States deals with a surge in new infections that is overwhelming hospitals in some states.