From The National Law Review:
In response to the global spread of the Omicron variant, the Centers for Disease Control and Prevention (CDC) shortened the timeline for the pre-departure COVID testing requirement to one day applicable to in-bound travelers to the United States. Per the CDC announcement, effective on after 12:01 am EST on December 6, 2021, “all air travelers, regardless of citizenship or vaccination status, will be to be required to show a negative pre-departure COVID-19 viral test taken the day before they board their flight to the United States.”
The CDC order clarified that airlines must assure all U.S. bound passengers, 2 years or older, must present one of following pre-departure:
A negative COVID-19 viral test result taken no more than 1 day before departure; or
Documentation of recovery from COVID-19 (i.e., a positive COVID-19 viral test result on a sample taken no more than 90 days before the flight’s departure from a foreign country and a letter from a licensed healthcare provider or a public health official stating traveler is cleared to travel).
Air passengers will also be required to complete, and present to the airline prior to boarding, a form attesting to their compliance of these requirements. The CDC FAQs clarify the 1-day period is 1 day, not 24 hours, before the flight’s departure (the first flight if itinerary includes connecting flights) in order to provide more flexibility. For example, a negative result for a test taken at 9 am on a Thursday would be acceptable for a flight departing at 10 pm on the following Friday.
Exemptions from these testing requirements are limited and arduous. CDC advises exemptions “may be granted on an extremely limited basis when emergency travel (like an emergency medical evacuation) must occur to preserve someone’s life, health against a serious danger, or physical safety and testing cannot be completed before travel.” In order to apply for an exemption, individuals must submit their request to the nearest U.S. Embassy or Consulate.
At this time, these testing requirements are not applicable to travelers arriving at U.S. land border crossing or seaports.