All travelers arriving to the United States from an international destination must be fully vaccinated and show proof of a negative COVID-19 test taken no more than one (1) day before boarding their flight or provide documentation of having recovered from COVID-19. Airlines are required to check each passenger's documentation and will deny boarding to those who do not comply or cannot show proof of a negative test result or recovery. Passengers are required to show negative results from a test for current infection or viral test. 

More detailed information on this requirement can be reviewed on the Centers for Disease Control and Prevention (CDC) website as well as on this CDC flowchart.