Surely Covid protection via the vaccine isn't influenced by your immune system. The vaccine efficacy has got to have a zero immunity baseline. No way Pfizer are going to claim (for example) 90% efficacy then put in brackets and small print (unless you are already elderly and have weakened natural immunity). Or is it the case that a 90% in my example is an average and in effect it's almost 100% in a healthy 35yo but way lower in a care home bound 85yo ?
It's the latter, an average. Vaccines variously train, prime or teach your immune system how to respond to a virus, so their efficacy depends entirely on the immune system.