The Centers for Disease Control published a notice on January 12, 2021 requiring air passengers to the U.S. to obtain a COVID test before departure. Specifically,
"Air passengers are required to get a viral test (a test for current infection) within the 3 days before their flight to the U.S. departs, and provide written documentation of their laboratory test result (paper or electronic copy) to the airline or provide documentation of having recovered from COVID-19. Airlines must confirm the negative test result for all passengers or documentation of recovery before they board. If a passenger does not provide documentation of a negative test or recovery, or chooses not to take a test, the airline must deny boarding to the passenger.
The order will become effective on January 26, 2021.
More information can be found here: https://www.cdc.gov/media/releases/2021/s0112-negative-covid-19-air-passengers.html