Abstract Background To protect infants aged <2 months against pertussis, the United States recommends Tdap (tetanus toxoid, reduced diphtheria toxoid, and acellular pertussis) vaccination during each pregnancy. Data are limited on the strategy’s effectiveness against pertussis in infants aged ≥2 months. Methods Pertussis cases aged 2-<6 months with cough onset between 1/1/2011-12/31/2014 were identified in six U.S. states. Controls were 2-<6 months of age, hospital-matched, and selected by birth certificate. Mothers were interviewed to collect demographic and healthcare information. Provider-verified vaccination history was obtained for infants and mothers. Adjusted odds ratios (aORs) were calculated using conditional logistic regression; overall vaccine effectiveness (VE) was estimated as (1 - aOR) x 100. To describe maternal Tdap VE modified by infant DTaP (diphtheria and tetanus toxoids, and acellular pertussis) doses, case-control sets were unmatched and a time-to-event analysis conducted through a Generalized Linear Mixed Model. Results A total of 335 cases and 927 controls were enrolled. The overall adjusted VE estimate for Tdap during pregnancy was 45.6% (95% confidence interval [CI], 5.8% – 68.5%) and increased slightly, but not significantly against infant hospitalization (55.7% (-116.8% – 90.9%)). Although point estimates were not significant, VE was modified by infant DTaP doses (58.8% (-6.0% – 84.0%) for zero DTaP doses, 30.5% (-21.4% – 60.2%) for one DTaP dose, and 3.2% (-170.8% – 65.4%) for two DTaP doses). Conclusions Our study suggests that there is some benefit of maternal Tdap vaccination beyond the first two months of life, however, on-time vaccination of infants remains critical to maintain protection from pertussis.