On January 12, 2021, the U.S. Centers for Disease Control (CDC) stated that CDC has issued "an Order requiring all air passengers arriving to the US from a foreign country to get tested no more than 3 days before their flight departs and to provide proof of the negative result or documentation of having recovered from COVID-19 to the airline before boarding the flight. This Order will go into effect on January 26, 2021."
A set of CDC Frequently Asked Questions clarifies that "This Order applies to all air passengers, 2 years of age or older, traveling into the US, including US citizens and legal permanent residents."
A January 12, 2021 CDC media statement that effective January 26, 2021 states: "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... This order was signed by the CDC Director on January 12, 2021 and will become effective on January 26, 2021."
On January 21, 2021, President Biden issued Executive Order on Promoting COVID-19 Safety in Domestic and International Travel, which directs relevant agencies to assess the January 12, 2021 CDC order requiring a negative COVID-19 test result for airline passengers traveling into the United States, and, based on that assessment, take any further appropriate actions and planning regarding air, land, and sea travel.
On January 25, 2021, the Department of State published a Fact Sheet titled, Negative COVID-19 Test Required for Travel to the United States Beginning January 26.