Pregnant women are randomized to receive an initial injection of VaxSyn or alum placebo between week 16 and week 24 of gestation, followed by monthly booster injections concluding at the end of pregnancy, for a total of five injections. Patients may have optional booster immunizations (vaccine or placebo) at 3, 6, 9, and 12 months after delivery. Mothers and infants are followed through 18 months after delivery.