breakthrough infections can happen for a bunch of reasons- i'm not sure if we actually know why it happens with covid, but it could be down to the biology of the person infected or a mutation in the virus.
Why do we have a 1st, 2nd and a booster vaccine and still get the virus? Any medical expert to make me understand?
"the condition of being immune; the protection against infectious disease conferred either by the immune response generated by immunization or previous infection or by other nonimmunologic factors. It encompasses the capacity to distinguish foreign material from self, and to neutralize, eliminate, or metabolize that which is foreign (nonself) by the physiologic mechanisms of the immune response."