Advertisement United Airlines is now requiring passengers boarding flights originating in the U.K. to show proof of a negative COVID-19 test taken no later than 72 hours prior to departure, said Annabelle Cottee, United Airlines spokeswoman. There is one daily nonstop flight from the London Heathrow to San Francisco. That flight will be suspended starting Jan. 1, a decision that was made by the flight’s carrier, United Airlines, before news of the variant form of the virus had been reported. United Airlines passengers wait in line to register at the COVID-19 rapid testing site at San Francisco International Airport in October.