 Study Subject: The inclusion criterion is to enroll mothers at the beginning of second trimester i.e. approximately 12 weeks of gestation, willing to stay in Dhaka during pregnancy and willing to admit in the clinic at delivery. The gestational age will be determined by self-reported LMP, which is likely be underestimated in most cases. A two-week variation will be acceptable in this proposed study. The exclusion criteria are: history of systemic disease, previous complicated pregnancies or of pre-term delivery, abortion, congenital anomaly, and hypersensitivity to influenza vaccine or receipt of the vaccine. 