Although immunization has decreased rates of childhood pertussis, immunity wanes over time, making pertussis an important cause of prolonged cough in adults. Furthermore, infected adults can transmit disease to unimmunized infants and children. To assess whether adolescent and adult pertussis immunization is decreasing disease effectively in these groups, Ward and colleagues conducted a prospective, randomized, double-blind trial of pertussis vaccination.

The multicenter study enrolled 2,781 healthy persons between 15 and 65 years of age. Participants were randomized to acellular pertussis vaccine or hepatitis A vaccine (for the control group). Of the participants enrolled in the study, 547 dropped out, mostly during the final six months of the trial. Participants were followed for up to 2.5 years with biweekly telephone calls for illness assessment, scheduled blood samples, and culture and serology for acute illnesses lasting more than five days. Trial completion was 94 percent at 12 to 17 months and 80 percent at 18 to 24 months. Primary pertussis infection was defined by a cough of more than five days with laboratory confirmation of infection through serologic or microbiologic evaluation. No serious vaccine-related adverse events occurred. Investigators evaluated 2,672 patients for cough of at least five days' duration. There was no significant difference in the incidence of cough-related illness between the study groups, and 50 percent of patients had no cough during the follow-up period.

Ten cases of primary pertussis infection occurred. These patients did not demonstrate any unique clinical characteristics other than adolescent age ( P = .027), longer cough duration ( P < .001), and fewer fevers ( P = .047). Nine of the 10 pertussis cases occurred in the control group, equating to 370 cases per 100,000 person-years. Although pertussis accounted for only a small part of the cough-related illness (0.7 percent), it did make up a larger proportion (5.7 percent) of coughs lasting more than 56 days.

The authors conclude that acellular pertussis vaccination in adolescents and adults effectively decreases pertussis infection in these groups.

editor's note: The Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention now recommends that adolescents and adults be immunized against pertussis. This will not change the frequency of recommended immunizations; instead, diphtheria booster shots currently scheduled for adolescents and adults will be replaced by a vaccine also containing acellular pertussis. —c.c.