Variants of the SARS-CoV-2 virus continue to emerge in countries around the world, and there is evidence of increased transmissibility of some of these variants. With Covid-19 infections and hospitalizations currently surging in the United States, effective January 26, 2021, the U.S. Centers for Disease Control and Prevention is expanding the requirement for a negative COVID-19 test to all air passengers entering the United States.
Air passengers are required to get a viral test for current infection within the 3 days before their flight to the U.S. departs, and provide written documentation of the laboratory test result (paper or electronic copy) to the airline. Documentation of having recovered from COVID-19 can be substituted for a negative test. This new testing requirement for air passengers will decrease, but not eliminate, the chance of new coronavirus variants entering the country before a large portion of the population is vaccinated.
Airlines must confirm the negative test result for all passengers or documentation of recovery before they board. If a passenger does not provide documentation of a negative test or recovery, or chooses not to take a test, the airline must deny boarding to the passenger.
In addition, the CDC recommendations, but at this point is not mandating, that new arrivals get tested again 3-5 days after arrival and stay home for 7 days post-travel.
Interestingly, being vaccinated against Covid-19 disease does not void the requirement for a negative test before traveling to the U.S. Current vaccines greatly reduce the chance of developing serious Covid-19 symptoms. They do not prevent infection by and potential transmission of SARS-COV-2 virus.
As far as stopping the introduction of new variants of the virus from abroad, pre-departure testing is better than nothing but insufficient to be 100% effective. That would require testing on arrival, quarantine, and a negative test before being released from quarantine. Thailand is one country with that type of process for travelers who arrive by air.