How long is an infant protected from the flu shot given to a pregnant woman in the 3rd trimester? i got the 4 virus version.

Hard to be sure: The antibodies built during pregnancy are pushed into baby at a level higher per pound than mom retains. That said, all such antibodies begin to age out and become less effective with time. By 3-6 mo there is little if any left. Since there are always a few more flu strains circulating than the ones they can put in the vaccine, it is important to avoid exposure as the primary form of prevention