You have to use "common sense" when you discuss all this stuff. Eight relatives were all different, in age, smokers, none smokers, drinkers, already in bad health, taking other medications, other pre existing conditions.
Should it surprise you their IMMUNE SYSTEMS all are different, not all will produce the same number of anti bodies.
As a poster asked you did they get the Delta a more severe form?
So one size does not fit all.
So my wife who got Delta, in May this year, was very bad and had a hard recovery,
She has not been recommended to get the vaccine yet by our Doctor, as her anti body counts are high right now. That is a result of her immune system kicking in and bringing them up the count of anti bodies. So by December she could get her first shot, but it may be unnecessary if her anti bodies and the right type of proteins are still in her system protecting her.
https://www.youtube.com/watch?v=NOK6drNPNck