Whooping cough is a difficult disease to control and is particularly serious in infants, especially those too young to vaccinate. This research investigates the effectiveness of providing pregnant women a dose of whooping cough vaccine to prevent or lessen the disease in their infants. Transfer of antibodies through the placenta in pregnancy is expected to offer some passive protection until the infant is protected by childhood immunisation starting at 6 weeks of age. The study will firstly use data for all births between 2011-2013, all infants who were diagnosed with whooping cough and whether or not their mothers received the vaccine in pregnancy. Knowledge from this research will help inform the New Zealand immunisation programme and better control of whooping cough.