Effective Jan. 26, 2021, the CDC will require all air passengers entering the United States provide written documentation of a negative COVID-19 viral test within 3 days of flight departure. CDC guidance and up-to-date information regarding this new testing requirement can be found here:

Requirement for Proof of Negative COVID-19 Test or Recovery from COVID-19 for All Air Passengers Arriving in the United States | CDC

In brief: If you have had a positive viral test in the past three months and you have met the criteria to end isolation, you may travel instead with documentation of your positive viral test results and a letter from your healthcare provider or a public health official stating you have been cleared for travel.

Airlines must confirm the negative test result or documentation of recovery for all passengers 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 will deny boarding to the passenger. Passengers must also attest, under penalty of law, to having received a negative qualifying test result or recovery from COVID-19 and medical clearance to travel.