The Science and Fiction of the “Resurgence” of Pertussis

During the last 2 decades an increase of reported pertussis has been noted in the United States and other countries in which infants and children are universally immunized.1–3 Presently, this “resurgence” of pertussis is presented with alarm in frequent newspaper headlines from many geographic areas throughout the United States.Reported pertussis in the United States in the prevaccine and vaccine eras is presented in Fig 1, and the age distribution of pertussis cases in a prevaccine period and 2 periods in the vaccine era are presented in Table 1. As can be seen (Fig 1), the “resurgence” is modest when the big picture is observed. The data in Table 1 note that in the prevaccine era 95% of the reported cases were in children <10 years old, with only 7.5% occurring in the first year of life. In contrast, in the 1978–1981 period, although the vast majority of cases still (88.2%) were reported in children <10 years old, the percentage of cases reported in infants was >6-fold higher than in the prevaccine era. Finally in the “resurgent” era 49.8% of the cases were reported in persons >10 years old.The modest increase in reported pertussis is of concern but hardly deserves the title of “resurgent” disease because the rate of illness in the United States is still 50-fold less than in the prevaccine era. There have been many suggestions as to the cause of the increased reporting of pertussis. These include: 1) genetic changes in Bordetella pertussis making vaccines less effective, 2) lessened potency of pertussis vaccines, 3) waning of vaccine-induced immunity, 4) greater awareness of pertussis, and 5) the general availability of better laboratory tests.Key to understanding pertussis is the recognition that the epidemiology of reported pertussis is different from the epidemiology of B pertussis infection.4–8 Specifically, reported pertussis is cyclic with peaks occurring on average every 3 years whereas B pertussis infection is endemic with regular transmission among adolescents and adults.Studies in the Netherlands have noted nucleic acid changes in the genes of pertactin and pertussis toxin.2,9 It has been suggested that these changes have led to respective amino acid changes so that present vaccines are less effective. Although these changes have been clearly documented, there is no laboratory or clinical evidence indicating decreased vaccine efficacy. Because there is considerable redundancy in B pertussis virulence factors, it is unlikely that vaccine-driven mutations are and will be troublesome in the future.Lessened potency of pertussis vaccines is a legitimate issue.4,10 Because of reactogenicity of whole-cell vaccines (diphtheria, tetanus, and pertussis [DTP]), vaccine manufacturers manipulated culture techniques in attempts to lessen toxicity. In the 1970s in Sweden these attempts led to an ineffective vaccine in 1979.11 In the United States it is also known that attempts were also made to lessen reactogenicity. The acellular vaccine trials in Sweden and Italy noted that the Connaught DTP vaccine used in the United States had minimal efficacy.12,13 In addition, 2 of the first 4 acellular pertussis component vaccines (diphtheria, tetanus, and acellular pertussis [DTaP]) available in the United States had poor efficacy.10 However, despite the use of many poor vaccines for a 10-year period there is little scientific evidence that this contributed to the “resurgence” of reported pertussis in the United States. In contrast, such evidence does exist in Canada.3Today most of the cases which have been reported in persons >10 years old are the result of waning immunity. However, waning immunity is a function of both vaccine- and disease-induced immunity. Contrary to popular belief, there is ample evidence today that disease-induced immunity is no more long-lasting than vaccine-induced immunity.4,5,8 Today B pertussis is circulating and causing disease in adolescents and adults of all ages and all have had previous infections and many past immunizations. There are about 1 million cases of pertussis in adolescents and adults in the United States each year and about 13% of all prolonged cough illnesses in adolescents and adults are attributable to B pertussis infection.8The modest increase in reported pertussis during the last 2 decades is mainly attributable to a greater awareness of pertussis and in particular the recognition of the occurrence of atypical disease in adolescents and adults. The study of pertussis vaccines, both DTP and DTaP, has led to more general studies of pertussis and its epidemiology. Resulting from this have been literally hundreds of papers on pertussis, which has led to greater awareness. Although most internists and others who care for adults still haven’t gotten the message, public health personnel have. This increased awareness of pertussis and experiences gained in the study of DTaP vaccines has led to better laboratory diagnosis of B pertussis infection. In general, B pertussis culture is now performed better, polymerase chain reaction is widely available, and adolescent and adult disease can be diagnosed by single-serum serology.Pertussis will continue to be a problem until we address the epidemiology of B pertussis. The coming availability of dTaP vaccines for use in adolescents and adults and their universal use is the only solution to the “resurgent” pertussis problem.

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