They are doing random tests on some American visitors. I would think if you test positive you would just return home. If you are an American citizen they can’t deny you entry back into USA no matter how sick you might be.
My landlord lives in Wisconsin and has attempted to cross the border 5 times in the last 2 months—- he was denied twice and that was because nobody at the border communicates very well with the Thunder Bay Health Unit. They can’t seem to agree on which paperwork is nessesary.
