In addition to the article quoted by Scubaman, here is another one that more fully explores the reasoning behind the changes in recommended use of the AstraZeneca vaccine. It is still a very safe vaccine and the primary one being used by the UK, although with changed age limits. It is also still heavily used by several other countries, such as Australia.

https://globalnews.ca/news/7962365/covid-coronavirus-vaccine-astrazeneca-canada-dose-2-uk/

As the article points out, there are three primary reasons why Canada's NACI has now opted to recommend either Pfizer or Moderna as the choice for the second dose for those who initially got the AZ dose. It is not so much that AZ is not an effective vaccine, but there have been some rare blood-clotting issues.

First, there have been supply issues with AZ, due to the troubled plant in Baltimore. Second, we now have a steady and assured supply of more than enough mRNA vaccines from both Pfizer and Moderna to cover all eligible Canadians with two doses. That gives us the luxury of picking which ones to recommend, which also avoids any blood-clotting issues. Third, several studies seem to show that a second shot of either Pfizer or Moderna actually provides a higher level of efficacy for those who initially had the AZ shot.

Initially all of our vaccines were sourced from Europe or India. With increased supply in the USA, combined with reducing demand, it is now possible and more logical for Moderna and Pfizer to fill their Canadian orders by shipping them from their US inventory, rather than from their offshore facilities. Until recently, the US Government did not allow any US-produced vaccines to be shipped elsewhere.

I think eventually the USA and other countries, including St Maarten, will allow those who have received the AZ vaccine to be eligible to enter, even if it is never formally approved by the FDA for domestic use. I suspect this will be part of the negotiations for some type of international agreement for a common and easily-recognized vaccine authentication ("passport") for travellers.