The stretch between birth and a baby's first vaccinations is the period when several common infections do their worst damage, and the only immune protection available during those weeks is what crossed the placenta before delivery. That biology is why obstetric groups recommend specific vaccines during pregnancy rather than after.
The timing gap is concrete. Infants do not begin their own pertussis vaccine series until around two months of age, and most pertussis deaths occur in babies younger than three months. Influenza vaccination cannot begin until six months. RSV season arrives regardless of how old a baby is when it starts.
