A baby will receive IgG against chicken pox from mom before delivery. This mom transfered antibody will decrease during the first year. The breastmilk will contant IgA which will protect the GI tract and possiblely parts of the upper respiratory tract as long as the baby breastfeeds. This along with other immune protective factors including whatever it is in milk that boost the baby's immune system will help protect against chicken pox. Unlike the baby will get chicken pox but not impossible.
It also helps to get the vaccine , Varivax, at one year.
Often I've wondered; if a baby is BF we know that protects against otitis but if the baby also has reflux is that added protection? Could the baby reflux breastmilk IgA and other factors up to the eustachian tube?
-Rob