I suspect that the vax weakened their immune systems. I also think isolation didn’t help either. You need to be exposed to stuff to keep your immune system functional.
My sister-in-law wasn’t isolated. She’s been more active than me! She’s got 5 grandkids that she babysits for often.
I think she got the booster. She’s older than me and takes care of her dad and is around my mom alot.
I’m guessing she got it from her grand kids. We did not hug and kiss, so maybe it’s not as transmissible as people claim.