One should assume that a nursing baby is affected by everything the mother puts in her body. Her body is making the protein of the milk out of her intake. The lipids in the vaccine are protein. They go in the baby. You don't say how old the baby is, but it isn't going to have a normal immune system to cope with foreign proteins and at this point I think there is only one trial about to start on mother's and infants. Why does she want a vaccine that doesn't really prevent infection, anyway? If she is adamant, try to get her to switch to formula at least for a few weeks. Point out that every veterinary medicine says "don't eat any animal or it's meat, milk, or eggs" for some period of time, usually 6-8 weeks, after dosing.