Compared with older children and adults, infants aged <12 months have substantially higher rates of pertussis and the largest burden of pertussis-related deaths. Since 2004, a mean of 3,055 infant pertussis cases with more than 19 deaths has been reported each year through the National Notifiable Diseases Surveillance System (CDC, unpublished data, 2011). The majority of pertussis cases, hospitalizations, and deaths occur in infants aged ≤2 months, who are too young to be vaccinated; therefore, other strategies are required for prevention of pertussis in this age group. Since 2005, the Advisory Committee on Immunization Practices (ACIP) has recommended tetanus toxoid, reduced diphtheria toxoid and acellular pertussis (Tdap) booster vaccines to unvaccinated postpartum mothers and other family members of newborn infants to protect infants from pertussis, a strategy referred to as cocooning (1). Over the past 5 years, cocooning programs have proven difficult to implement widely (2,3). Cocooning programs might achieve moderate vaccination coverage among postpartum mothers but have had limited success in vaccinating fathers or other family members. On June 22, 2011, ACIP made recommendations for use of Tdap in unvaccinated pregnant women and updated recommendations on cocooning and special situations. This report summarizes data considered and conclusions made by ACIP and provides guidance for implementing its recommendations.