According to the CDC, air passengers entering the U.S. will be required to present documentation of a negative COVID-19 viral test (called a Qualifying Test) conducted within the 3 days before their flight to the United States departs.
Documentation should be for a viral test as approved or authorized by the relevant national authority.
Test Results Must State: Negative, SARS-CoV2 RNA Not Detected, SARS-CoV-2 Antigen Not Detected, or COVID-19 Not Detected.
Documentation may be paper or electronic copy and must include the name and contact information for the laboratory or healthcare personnel who performed the test.
Test results may be required to be shown again upon landing.
Alternatively, passengers may provide documentation of having recovered from COVID-19 with documentation of test results conducted during the three months preceding the flight’s departure from a foreign country.
Test results must state: Positive, SARS-CoV2 RNA Detected, SARS-CoV-2 Antigen Detected, or COVID-19 Detected. Invalid will not be accepted.
Additionally, a letter from a licensed health professional stating that the passenger has been cleared for travel (called Documentation of Recovery).
Finally, all airline passengers will be required to present a simple CDC attestation form to the airline for collection prior to boarding. That form can be found here for reference.
For more information, you may reference the FAQs on the CDC website, which can be found here.