Impact of the US Maternal Tetanus Toxoid, Reduced Diphtheria Toxoid, and Acellular Pertussis (Tdap) Vaccination Program on Preventing Pertussis in Infants 2 to <6 Months of Age: A Case-Control Evaluation.

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 case infants aged 2 to <6 months with cough onset between 1 January 2011 and 31 December 2014 were identified in 6 US states. Controls were 2 to <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)×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 was 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% to 90.9%]). Although point estimates were not significant, VE was modified by infant DTaP doses (58.8% [95% CI, -6.0% to 84.0%] for 0 DTaP doses, 30.5% [-21.4% to 60.2%] for 1 dose, and 3.2% [-170.8% to 65.4%] for 2 doses). Conclusions: Our study suggests that there is some benefit of maternal Tdap vaccination beyond the first 2 months of life, however, on-time vaccination of infants remains critical to maintain protection from pertussis.