travelers sneaking through the cracks in the system....even Canada's draconian response toward "slowing the movement of virus" is (exceptionally) full of holes, ill thought-out, and frankly, proving to be of little benefit.
Every Nation's "response" has to be situational (even the Three Amigos), but carried-out operationally under the precautionary principle (being proactive, not reactive).
The huge issue plaguing the Three Amigos (and every other nation on Earth), is that some nations (4 of them out of 200+ Nations) actually HAVE vaccines in great quantities, while others have almost none. So, situationally, you have a huge imbalance country-to-neighboring-country: vaccinated to non vaccinated. How would the neighboring non vaccinated (and, possibly economically underprivileged, like Mexico) countries react? Totally shutting their borders, and cancelling 100% of passenger air traffic? Drilling down to the microscopic, RVers free flow across the Three Amigos borders, a bi or tri-lateral agreement was never meant to address dynamic imbalances like a pandemic delivers. Few if any Nations on Earth ever had any effective pandemic plan. There are no easy fixes, nor easy answers.
A global concerted pandemic response needed a global pandemic response "law" enforceable. This does not exist.