Abstract and Introduction
Abstract
Background: Pertussis vaccination has reduced the number of notified cases in industrialized countries from peak years by more than 95%. The effect of recently recommended adult and adolescent vaccination strategies on infant pertussis depends, in part, on the proportion of infants infected by adults and adolescents. This proportion, however, remains unclear, because studies have not been able to determine the source case for 47%-60% of infant cases.
Methods: A prospective international multicenter study was conducted of laboratory confirmed infant pertussis cases (aged ≤6 months) and their household and nonhousehold contacts. Comprehensive diagnostic evaluation (including PCR and serology) was performed on all participants independent of symptoms. Source cases were identified and described by relationship to the infant, age and household status.
Results: The study population comprised 95 index cases and 404 contacts. The source of pertussis was identified for 48% of infants in the primary analysis and up to 78% in sensitivity analyses. In the primary analysis, parents accounted for 55% of source cases, followed by siblings (16%), aunts/uncles (10%), friends/cousins (10%), grandparents (6%) and part-time caretakers (2%). The distribution of source cases was robust to sensitivity analyses.
Conclusions: This study provides solid evidence that among infants for whom a source case was identified, household members were responsible for 76%-83% of transmission of Bordetella pertussis to this high-risk group. Vaccination of adolescents and adults in close contact with young infants may thus eliminate a substantial proportion of infant pertussis if high coverage rates can be achieved.
Introduction
Bordetella pertussis continues to circulate and cause disease even in populations with high vaccination coverage of infants and children. Waning of vaccine-induced immunity is cited as an important factor contributing to this persistent problem. Despite widespread vaccination coverage (ie, 95% for the primary series), the reported incidence of infant pertussis in the United States tripled in the past 2 decades from 34.2 cases/100,000 in the 1980s to 103.5 cases/100,000 infant population in 2003. Infants too young to have completed their primary vaccine series account for the majority of pertussis related complications, hospitalizations and deaths.
One strategy for protecting infants is to increase herd immunity by vaccinating close contacts. France was the first country to introduce a booster dose for adolescents in 1998, followed by Germany, other European countries, Canada, Australia and Japan. France and Germany have also recommended a booster for parents and health care workers in contact with young children. The United States Advisory Committee on Immunization Practices (ACIP) recently recommended that all adolescents and adults receive a single booster dose of Tdap (tetanus toxoid, reduced diphtheria toxoid and acellular pertussis vaccine) in place of Td (tetanus and diphtheria toxoids vaccine). Modeling has shown that the effect on infant pertussis of routinely vaccinating all adolescents and adults critically depends on the proportion of infants infected by family members.
Several studies have investigated the transmission of pertussis within households using a variety of study designs: case control studies in outbreak settings, investigations using general and hospital-based surveillance data, and secondary analyses using vaccine efficacy trial data. These studies reported that parents (20%-48%) and siblings (19%-53%) were common sources of infection for infants for whom a source was identified. However, interpretation of these findings is subject to important limitations. In these studies, the primary source of infection could not be identified in 47%-60% of infant index cases. Further, except in one small study, the diagnosis of the index case was not always laboratory confirmed, and identification of source cases relied predominantly on a clinical diagnosis of pertussis. It remains therefore unclear who transmits pertussis to young infants.
This study was designed to determine more precisely who transmits B. pertussis to young infants. We collected comprehensive clinical and biologic diagnostic data on close contacts to infant cases with laboratory confirmed pertussis.