12/28/2020:The U.S. government has imposed a new COVID-19 testing requirement for people traveling to the U.S. from the United Kingdom, including US citizens and legal permanent residents. The CDC (Centers for Disease Control and Prevention) has posted an FAQ page which explains this new requirement. Note that the new requirement is in addition to the existing physical presence ban limiting travel from the U.K., but F-1 students, and J-1 students who have obtained a national interest exception (NIE), can still enter the United States, with proof of a negative COVID-19 test.