I'll be flying up to Khon Kaen from Bangkok next week. Both my wife and I have each had two vaccinations as of December. Is there any requirement to prove a negative swab test flying into Khon Kaen or a week later back to Bangkok?

Simply: NO test required. Vaccination certificate fully sufficient.

To Khon Kaen unlikely to be asked for vax. cert. (might or might not).

Back to Bangkok not even vaccination certificate required by the rules.

I did not have to show anything in October/November and December/January (AirAsia/ThaiSmile) both ways.

 

When entering and exiting KKC airport passport/IDs were scanned.