How can you explain the science behind the need for vaccines?

As a baby grows inside its mother’s womb, it is provided directly from the mother via the placenta with massive amounts of immune-protecting antibodies. So, yes, the stronger the mother’s immune system is, then the stronger the infant’s inherited immunity will be.

While the fetus grows, its own immune system learns to remember all its own body molecules as Itself (which is why, for example, transplanted organs are rejected without medication). Once a baby is born (with the exception of extremely premature babies), the baby’s body immediately recognises it is no longer inside the sterile environment of the womb and kick-starts its lifelong task of protecting itself from the germs it is now covered in — as even the cleanest of postnatal environments has plenty of germs.

The infant’s inherited immune system is passive and provides no long term immunological memory, and so it slowly wanes over the months after birth as the baby begins to develop its own active immunological memories. (Breastfeeding continually boosts and stimulates an infant’s passive immune system in their first year — however the bulk of its initial immunity is inherited during the final trimester in the womb.)