In a previously reported double blind placebo-controlled trial it was shown that vaccination with pertussis toxoid during infancy reduced the incidence of pertussis in the vaccinees. Parents and siblings of participants in the trial were followed for pertussis to determine whether vaccination provided indirect protection of close contacts in a nonvaccinating country with a high incidence of pertussis.
A group of 3450 infants were randomized to vaccination with diphtheria, tetanus and pertussis toxoids (DTPtxd) or to diphtheria and tetanus toxoids (DT). Pertussis cases were actively sought and diagnosed by cultures and serology in vaccinees (previously reported) and in family members during 2 years after the third vaccination.
Pertussis as defined by the World Health Organization (paroxysmal cough of ≥21 days and certain laboratory criteria) was diagnosed in 11 parents of DTPtxd recipients and in 26 parents of DT recipients; indirect protection was 60% (95% confidence intervals, 16 to 82%). In nonvaccinated younger siblings of DTPtxd and DT recipients there were 10 and 18 cases of pertussis, respectively; indirect protection was 43% (95% confidence intervals, −31 to 76%). When all cases of pertussis with cough ≥7 days were included, the indirect protection was 44% (95% confidence intervals, 7 to 67%) in parents and 56% (95% confidence intervals, 9 to 81%) in younger siblings.
Vaccination of children with pertussis toxoid reduces spread of pertussis to close contacts, which suggests that mass vaccination with pertussis toxoid would induce herd immunity.