According to the CDC’s testing order, “Airlines must confirm the negative test result for all passengers before they board. If a passenger chooses not to take a test, the airline must deny boarding to the passenger.”
The CDC also recommends getting tested three to five days after you arrive in the US, and self-quarantining for seven days.
Whether this new rule will have any significant impact, however, is another matter. According to CNN, researchers on the CDC’s COVID-19 response team say that testing three days before a flight will only reduce viral transmission by just five to nine percent, while testing the day of departure “reduces the risk of transmission by 37 to 61%, according to the CDC scientists’ November report.” Dr. Paul Offit, an infectious disease specialist at the University of Pennsylvania, compared it to “putting up a chain link fence to keep out a mosquito.” 