I yesterday received my booster (Pfizer) at a walk-in vaccination center set up at the provincial hospital in the Changwat where I live.
A no fuss, we’ll managed process, I was in and out in under 50 minutes including observation.
Documents required, passport and vaccination certificate (though I believe MorProm would have sufficed if I had not brought my previous certificate as it was the reference number they needed).
As for side effects, I have a mild dull ache at the injection site, sufficient I feel to excuse me from household chores but not so bad as to keep me from cycling today and the weekend.
A big thanks to the people in the Thai health services and administration who are providing this essential service.