Can you elaborate? What was the change?

I am quite curious too. I had heard that, despite arm vascular being very consistent among individuals, it does still vary. And I think for most vaccines I guess it doesn't matter if you hit something other than muscle. Maybe for the mRNA vaccines it does matter? I'm baselessly speculating though. Wish other person hadn't been so vague.

Not 100% sure but I think it was this: Never inject into a vein. Always inject into a muscle.

In an intramuscular injection this is always tested by the nurse and this complication is far rarer than a nerve injury by the syringe. But in billions of cases anything can happen.

Yes, they mandated a test to check if it was being injected into a vein.

Pretty common test as well, pull the plunger back on the syringe if blood comes up you hit a vein.

A lot of drugs have this test, particularly drugs that should be injected in fat or muscle.