The antibodies a newborn baby receives across the placenta and in its mother's milk provide

Explanation

The baby is given ready made antibody rather than making its own, so the protection is passive, and because it comes through a natural route rather than an injection it is natural. It covers the first few months while the infant's own immune system is still immature, then disappears, which is why vaccination begins early. An injection of antitetanus serum is the artificial version of the same thing.