Epidemiology of pertussis and reactions to pertussis vaccine.

It remains clear that pertussis is a dangerous infectious disease that is well-controlled in industrialized countries by widespread immunization. In the developing world, it remains a source of high morbidity and mortality because of previously inadequate immunization programs. However, because of the intense efforts of the World Health Organization's Expanded Programme on Immunization, the effects of pertussis have already been ameliorated and show promise of being within a decade of approximating the situation in the developed world. Pertussis can be controlled only by immunization; other measures such as antimicrobial therapy offer negligible benefit. A problem that has been addressed in recent years is the excessive reactivity of whole-cell pertussis vaccine, which undoubtedly includes components of the organism that are irrelevant to the induction of immunity and are excessively reactive. Although epidemiologic studies appear to have largely, if not completely, absolved pertussis vaccine of responsibility for inducing death or permanent neurologic disability, a less reactive vaccine is highly desirable, not only to promote acceptance of a full course of immunization for the world's children but also for simple humanitarian reasons. Additionally, it has become evident that, because of waning immunity, pertussis increasingly occurs in adults. A less reactive vaccine would offer opportunity for reinforcement of immunity beyond childhood. The development of better, though as yet incomplete, understanding of the biology of Bordetella pertussis and its relation to humanity offers the opportunity for the production of less reactive vaccines free of irrelevant components. Acellular pertussis vaccines have been used exclusively in Japan for more than 10 years, and one such preparation, combined with diphtheria and tetanus toxoids, was licensed in the United States in late 1991 for use as the fourth and fifth doses of DTP, given at 15 months and prior to school entry. Field trials of this and other acellular DTP preparations are currently under way to determine their clinical efficacy in infants. It is probable that, within a very few years, whole-cell pertussis vaccine will be replaced by these newer preparations and that, in addition, the acellular product will be combined with other antigens, such as Haemophilus influenzae type b vaccine.