No shots...not getting in.

I agree with the posters above that I don't think there is going to be a flood of people crossing the border with faked vaccination documents. But this is going to be an increasingly a requirement for travel, just like Yellow Fever vaccines are required for many parts of the world, and thus it is worthwhile to get out in front of the problem and figure out some universal way to demonstrate you are vaccinated.

The fact that there already appears to be a market for fake vaccine cards, does seem to indicate that people are planning on using them.