I would think 'proof of vaccination' could be more easily forged than a passport, and therefore cheaper on the Darknet, but I've not actually looked. I don't do airplanes, and not too inclined to go outside the USA anyway, so not something I'd likely need. Mexico (Tijuana) is as far as I've been, or likely to go, and since I got a lifetime supply of Mercurochrome last time, and since the Mexican Mafia seems in control of Tijuana, I doubt I'll be returning.