I don't have any info , on what would be required for an American to cross the Canada /US border this summer , but as of Feb. 14 , 2021. even Canadians returning from the USA , are required to have a negative Covid-19 test less than 72 hours old to cross back into Canada .
I would expect , IF and that's the BIG IF , any travel from the lower 48 to Alaska, by other than an Alaska resident returning home , there would be requirements at the border for , negative test or proof of vaccination ; as well as the time limit on travel through Canada.
I've sent a request to the federal government for a link to any border crossing requirements , BUT given the short term nature of border closure extensions , I don't expect they have given any thought to what might happen three months from now.
70% vaccination ( herd immunity ) won't happen here in Canada till September AT THE EARLIEST, so summer 2021 travel probably won't happen.