Upvote:-3
A - you're wrong about the land border. I(resident alien)'ve crossed a ton of times and even back when Canada had the vaccine requirement and you had to fill out your ArriveCan declaration, the US border guards never asked anything Covid related.
B - even for flights, the only proof I've ever shown in the US is a badly lit photo of my paper CDC vaccine card on my phone (that was filled in by me in the 1st place, because I did it early when the vaccination center was swamped, so the staff gave me a blank card and told me to fill it in myself, because they were too busy). There's no verification. This isn't like Canada that did spot checks in advance through ArriveCan. A photo of a physical card(don't know what province you're from, but BC had paper cards before the app, use that or the equivalent) + a bit of Photoshop and viola. I know it's frowned upon to suggest illegal actions, but vaccination status verification was a farce in the US even during the depths of the Pandemic and it would be silly to miss the trip due to something like this.
Upvote:10
In the US, other than a few limited exceptions, contrary to what happens in most other countries, all passengers have to enter the US (go through passport control, retrieve bags, go through customs), whether their final destination is the US or they are in transit.
So the rules for all passengers boarding a flight to the US are the same, you need all documentation required to enter the US.