IMMUNIZATION OF CHILDREN WITH PERTUSSIS TOXOID DECREASES SPREAD OF PERTUSSIS WITHIN THE FAMILY

Objective. In a previously reported double blind placebo-controlled tr ial it was shown that vaccination with pertussis toroid during infancy reduced the incidence of pertussis in the vaccinees. Parents and sibl ings of participants in the trial were followed for pertussis to deter mine whether vaccination provided indirect protection of close contact s in a nonvaccinating country with a high incidence of pertussis. Stud y design. A group of 3450 infants mere randomized to vaccination with diphtheria, tetanus and pertussis toxoids (DTPtxd) or to diphtheria an d tetanus toxoids (DT). Pertussis cases were actively sought and diagn osed by cultures and serology in vaccinees (previously reported) and i n family members during 2 years after the third vaccination. Results. Pertussis as defined by the World Health Organization (paroxysmal coug h of greater than or equal to 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 recipi ents there were 10 and 18 cases of pertussis, respectively; indirect p rotection was 43% (95% confidence intervals, -31 to 76%). When all cas es of pertussis with cough greater than or equal to 7 days were includ ed, the indirect protection was 44% (95% confidence intervals, 7 to 67 %) in parents and 56% (95% confidence intervals, 9 to 81%) in younger siblings. Conclusion. Vaccination of children with pertussis toroid re duces spread of pertussis to close contacts, which suggests that mass vaccination with pertussis toroid would induce herd immunity.