The US Transportation Security Administration has announced that non-US citizens & nonimmigrant passengers flying into the US between January 9 – April 10, 2023 must both:
a) present documentation of being fully vaccinated against Covid-19 (or present documentation that they are an Excepted individual) and
b) complete an attestation form confirming such. This process would occur during the check-in process ahead of flight departure to the United States.
Travelers who are not fully vaccinated or considered an Excepted passenger will not be permitted to board.
For more information please visit Requirement for Proof of COVID-19 Vaccination for Air Passengers | CDC.
For impacted travelers, there is a Travel Assessment link in the middle of the page (on the link above) that serves as a resource to help a traveler determine if they are able to fly to the US.