Estimation of pertussis vaccine efficacy in the presence of covariates in three randomized trials.

We estimated efficacy of pertussis vaccines in three randomized controlled trials with adjustment for several baseline covariates: presence of one or more other children in the household, sex of the study child, and geographical area. Adjusted and unadjusted efficacy estimates differed only trivially. We also assessed the association of efficacy with time since vaccination and background pertussis incidence. The acellular vaccines, except for the two-component vaccine in the Stockholm trial, appeared to maintain their efficacy during two years of follow-up. In contrast, efficacy of a whole-cell vaccine decreased significantly in both the Stockholm and Italian trials. The relationship between efficacy and background incidence was not consistent across studies and vaccines.