Also, the current outbreak was incubated in Mennonite communities before spreading wider. Despite living in Canada for over 100 years Mennonites are still considered immigrants by many because they are insular and different.

In other words, it's a racist talking point that has enough truth to make it non-trivial to challenge, but has no actual basis.

Changing the rate of immigration won't make a bit of difference for a disease like this. It only takes one infection in a susceptible population, and then it's game over. Unless your controls are 100% effective (including tourists and returning citizens), the only way to prevent an outbreak is for the vast majority of your people to be uninfectable.

An insufficiently vaccinated population is like a can of food. It might take longer to spoil if there's a pinhole in the side versus if you just crack it open, but it will spoil if there's any breach whatsoever.

So unless you propose to isolate to a degree more extreme than North Korea, blaming immigrants is just bigotry.