BACKGROUND A household contact substudy was performed as part of a prospective, cohort pertussis vaccine efficacy trial in Germany. METHODS Infants received four doses of either the Lederle/Takeda acellular pertussis component diphtheria-tetanus toxoids (DTP) vaccine (DTaP) or Lederle whole-cell component DTP vaccine at 3, 4.5, 6, and 15 to 18 months of age (Wyeth-Lederle Vaccines and Pediatrics, Pearl River, NY). An open control group received three doses of diphtheria and tetanus toxoids vaccine (DT) at 3, 4.5, and 15 to 18 months of age. Vaccine efficacy rates were calculated using a number of principal and ancillary case definitions for primary, secondary, and noncases by analyzing secondary attack rates in study infants after exposure to pertussis in the household using 7- to 28- and 7- to 42-day postexposure observation periods and the inclusion and the exclusion of noncases who received macrolide antibiotics or trimethoprim-sulfamethoxazole during the exposure period. RESULTS During a 3.5-year study period, 10271 infants (DTP or DTaP, n = 8532; DT, n = 1739) were enrolled and actively followed along with all household members for cough illnesses. Depending on the case definition, 160 to 519 household exposures to pertussis were identified. In general, secondary attack rates in DT recipients were low and this was primarily because of the frequent use of antimicrobial prophylaxis. Using the principal case definitions and the exclusion of noncases who received macrolide antibiotics or trimethoprim-sulfamethoxazole during the exposure period and the 7- to 42-day observation period, the efficacy of DTP against cough illness of greater than or equal to 7 days duration caused by Bordetella pertussis was 84% (95% confidence interval [CI] = 65-93) and that of DTaP was 58% (95% CI = 30-75). Using similar criteria, the efficacy against typical pertussis (greater than or equal to 21 days of cough with either paroxysms, whoop, or posttussive vomiting) was 94% (95% CI = 77-99) and 86% (95% CI = 62-95) for DTP and DTaP, respectively. The efficacy against any cough illness (with or without) laboratory confirmation was 54% (95% CI = 32-69) and 38% (95% CI = 13-56) for DTP and DTaP, respectively. CONCLUSIONS This household contact substudy within our cohort study, with active investigator-generated surveillance, was a severe test of vaccine efficacy. Both vaccines (DTP and DTaP) are better at preventing typical pertussis than mild illness. When case definitions similar to those in other recent trials are used, the Lederle/Takeda vaccine has an efficacy similar to other multicomponent DTaP vaccines.