The United States now requires Covid-19 RT-PCR testing before entering borders.
Certain states such a New York require all visitors to quarantine for 3 days and then require a negative RT-PCR test. More CDC info here
Many other countries and states also require a negative COVID-19 PCR certificate prior to boarding and when entering new borders.
The tests are typically required to be taken within 48-72 hours of travel. (This can vary by country or state).
Tests must be a RT-PCR (Antigen, Serology, or other Rapid tests are often not accepted by border officials).
Majority of airlines accept RT-PCR test as proof of travel; however, the tester is responsible to check on testing requirements of their destination and whether they accepts this test. We run a CLIA certified, high complexity lab that is approved by FDA under EUA.