The current CDC "Covid 19 Vaccination Record Card" is a cheapo thing and easy to duplicate. But it would not be hard for CDC or social security to come up with something more elaborate: multi-color and with a hologram. And, really, that's all you need. Sure criminal bands could counterfeit these things, but it's not particularly likely. Why spend $100 for a fake vaccine card, when you can just go in and get vaccinated, and earn a freebie? I really don't think counterfeiting is likely to be a big issue. And for the border, they already have a giant database at their disposal, so not hard to do a crosscheck. We need to get this done before the start of ski season!