This post is with regards for travel leaving Chiang Mai to Los Angeles, USA. Not sure what exactly is required for travel to any other country. I've not had any vaccinations. Days before leaving I had spoken with a JAL spokesperson and was told that an ATK (Antigen) test taken within 24 hours prior and the supporting health certificate would suffice to be allowed to board the aircraft. For fully vaccinated the time allowed was a more generous 72 hours. I was very glad to hear that the ATK test would suffice as it comes at a fraction of the cost of the PCR test. For the record there is a small professional clinic on the 3rd Floor of Suvarnabhumi Airport which one can take the ATK test for 550 Baht. This includes the supporting Health Certificate.
Wednesday Dec 1, traveling Chiang Mai to Bangkok I was not required to show my test results to board the Bangkok Air flight.
The following morning Dec 2, flying on JAL I traveled Bangkok to Los Angeles with a short stop in Narita, Japan. My ATK (Antigen) test was taken at a small clinic prior to leaving Chiang Mai. The supporting document which stated I showed Negative for Covid was given to the JAL/Thai Airways agent upon check-in, after which I was cleared for boarding.
When checking in with JAL I was given a short form to complete which clarified my vaccination status. I was told it would be collected prior to boarding the plane to L.A. in Japan. Other than that form there was no request for proof of anything during the layover/plane change in Narita.
I arrived in Los Angeles less than 24 hours ago, and with no further requirements needed I happily left the airport.
All in all, just basic proof of negative Covid status seemed to be all that was required at any given point.