Vegans can receive transfusions because I believe it’s down to “consent”. Humans consent to the blood draw. Crabs do not.
So vegans shouldn’t be taking any medicine or vaccines that have had blood crab blood used in their manufacture.
I have discovered the solution to the vegan problem! Tell all vegans about medicine being made using animal blood and they will die out faster than their B12 & iron deficiency does at the moment!