The original post: /r/askscience by /u/oofyeet21 on 2025-03-04 14:59:23.

So just to use chicken pox as an example, from what i understand the vaccine works by introducing an inactive strain of the virus into the body, which lets the immune system identify it, recognize it as a threat and develop countermeasures in order to immediately destroy it if the virus is encountered in the future. This protection lasts your entire life and never needs any sort of booster. I also understand that if a mother is vaccinated against chicken pox, then her baby will be protected by the remnants of her immune system for roughly the first six months of life, which is why we vaccinate infants around that time, as that temporary protection is wearing off. My question is: why does that protection wear off in the first place? If one instance of the dead virus is enough for my immune system to remember chicken pox and know how to kill it for the rest of my life, why does a baby’s immune system, which remembers chicken pox and knows how to kill it, suddenly forget how to fight it? What prevents it’s body from retaining it’s mother’s immunities, considering those immunities are already present within it?