I had someone ask me about the flu vaccine and breastfeeding today and thought I'd do a quick little post for you preggos. For the record, I am not saying you should or shouldn't vaccinate and when, just sharing what information I know. You might choose to do a little more research and/or talk to your doctor....
Vaccines primarily generate IgG antibodies in your system (to "remember" the virus and mount a quicker attack if it shows up later on). IgG antibodies pass to the baby through the placenta quite readily and these antibodies stay with baby and offer some protection to things mom is already immune to up to the first 6-8 months of life. Then the IgGs drop off and do not again reach levels similar to an adult's until the child is 7-8 years old. IgG antibodies are present in breastmilk, but in very low quantities. Vaccinating yourself does not confer immunity to the baby through breastfeeding.
The antibodies that are present in significant quantities in breastmilk are IgA antibodies. IgA antibodies are what ramp up when you are actually sick. So once you have been exposed to something and are fighting an active infection, you pass these antibodies to the baby through breastmilk and help them fight/prevent the infection as well (as a side note, if the baby has been exposed to an illness, your body has the ability to produce IgA antibodies specific to that illness in the breast--it all happens locally to help protect the baby regardless of whether or not you have had a systemic exposure).
The take home message here is that vaccinating yourself (against anything, but obviously, lots of people are wondering about the seasonal and H1N1 flu vaccines) will not give the baby immunity through breastfeeding (oh, how wonderful it'd be if it did!! That'd mean they would already be immune to anything we had become immune to through illness or vaccination!). I do not know the specifics of vaccination during pregnancy---how long it takes your system to produce those IgGs, how quickly they pass through the placenta, and how much protection that gives the baby. I realize my wording makes it seem like there could be a huge benefit to vaccinating during pregnacy. Keep in mind that there are risks and benefits to everything. It's up to each family and their doctor weigh those risks and benefits and make the decision that's right for them, I'm just passing on the information I know. Like I tell childbirth clients, anytime someone tells you there are NO risks or there are NO benefits to something, you should be skeptical---there are exceedingly few pregnancy/childbirth-related things I can think of that truly offer only possible good or only possible bad.
And while we're here, take the time to re-read my reminder about visiting with newborns during cold and flu season.