EXPERIENCE AND REASON |
From the Department of Pediatrics, David Geffen School of Medicine, University of California, Los Angeles, Los Angeles, California
| ABSTRACT |
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Studies of prolonged cough illnesses in adolescents and adults reveal that 13% to 20% are a result of B pertussis infection. Serologic studies suggest that the rate of B pertussis infection in adolescents and adults is
2.0% per year. The rate of cough illnesses (pertussis) caused by B pertussis infection in adolescents and adults is between 370 and 1500 per 100 000 population. These data suggest that there are between
800 000 and 3.3 million cases per year in the United States.
The coming availability of adolescent- and adult-formulated diphtheria and tetanus toxoids and acellular pertussis vaccines for adolescents and adults and their widespread use should reduce the reservoir of B pertussis disease. It is suggested that a universal program of adolescent and adult boosters would decrease the circulation of B pertussis in these age groups and possibly could lead to the elimination of the organism from the population.
Key Words: pertussis Bordetella pertussis adult pertussis adolescent pertussis pertussis epidemiology
Abbreviations: DTaP, adolescent- and adult-formulated diphtheria and tetanus toxoids and acellular pertussis ELISA, enzyme-linked immunosorbent assay PCR, polymerase chain reaction PT, pertussis toxin FHA, filamentous hemagglutinin PRN, pertactin FIM, fimbriae DTP, diphtheria and tetanus toxoids and pertussis APERT, adult acellular pertussis vaccine efficacy trial
In the prevaccine era pertussis epidemics followed a cyclic pattern, with peaks every 2 to 5 years.14 In the present vaccine era the overall incidence of pertussis has been reduced dramatically, but the cyclic pattern has not changed. This pattern contrasts with other infectious diseases that have been controlled by immunization. For example, with measles, as vaccine use became widely used, there was a lengthening of time intervals between cycles and an overall reduction in disease incidence.3,5
These 2 different patterns suggest that with measles both the disease incidence and the circulation of the virus have been reduced. With pertussis, however, the incidence of reported disease has been reduced, but the circulation of Bordetella pertussis has continued in the overall population in a manner similar to that which occurred in the prevaccine era.
Data generated during the last 15 years suggest that the circulation of B pertussis is occurring in adolescents and adults and is manifest by prolonged cough illnesses, which most often go unrecognized as pertussis.1,2 The understanding of the epidemiology of B pertussis infection is critical at the present time, because new adolescent- and adult-formulated diphtheria and tetanus toxoids and acellular pertussis component (DTaP) vaccines will soon be available in the United States.6,7 The optimal use of these vaccines will require an understanding of the epidemiology of B pertussis infection.
In this article, the epidemiology of reported pertussis (the disease) will be compared with selected aspects of B pertussis infections (recognized or unrecognized) in adolescents and adults.
| METHODS |
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The B pertussis infections have been determined by culture, polymerase chain reaction (PCR), antigen detection (direct fluorescent antibody), and most often by significant antibody-titer rises or high single-serum antibody values to pertussis toxin (PT) and other B pertussis proteins (filamentous hemagglutinin [FHA], pertactin [PRN], and fimbriae [FIM]) by ELISA or agglutination.
| RESULTS |
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In contrast with the prevaccine and early vaccine era, the reporting of pertussis in adolescents and adults has increased dramatically.1,8 As noted in Fig 2,
50% of the reported pertussis cases were in persons >10 years old. Since 1984 there has been a modest but steady increase in reported cases in the United States.8 In 2002 there were 9771 reported cases, for a yearly rate of 3.4 per 100 000 population.9 As noted in the introduction, in the vaccine era reported pertussis still has a cyclic pattern, and the peaks still occur at 2- to 5-year intervals.
In recent years there has been much concern in the lay and scientific literature about the "resurgence of pertussis." This resurgence was discussed recently in a Pediatrics commentary.8 To summarize, there are 5 possible causes for the increased reporting of pertussis: (1) genetic changes in B 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 in some areas of the country. Of these possible causes, it was felt that that the most significant factor was a general greater awareness of pertussis that has occurred in recent years. However, the observation of multiple outbreaks of pertussis in preadolescent and adolescent school populations suggest that, in my opinion, waning vaccine-induced immunity caused by vaccines that were less efficacious than in the past has also been a contributor.
Specifically, before the introduction of and universal use of DTaP vaccines in the United States there were 2 whole-cell pertussis component diphtheria and tetanus toxoids and pertussis (DTP) vaccines available. Both of these vaccines in the mid 1980s were immunogenic, but 1 of the 2 subsequently was found to be poorly immunogenic and had poor efficacy.1012 In contrast, the other DTP vaccine remained highly immunogenic and efficacious.13,14 In conjunction with the DTaP-vaccine efficacy trials in the early 1990s, it was found that all DTP vaccines except the Connaught USA vaccine were more efficacious than the comparative DTaP vaccines. Of the 3 DTaP vaccines in use in the United States today, 2 had less efficacy than a comparative DTP vaccine.15,16 The use of lesser potency vaccines from the late 1980s until the present may well be a contributing factor in the rise in preadolescent and adolescent pertussis.
The Epidemiology of B pertussis Infections
During the last 2 decades a number of studies have provided data that are useful in the delineation of B pertussis infection from reported pertussis. These studies include the study of prolonged cough illnesses in adolescents and adults, serologic studies over time used to determine infection rates in populations, and studies in defined populations to determine rates of symptomatic infections in adolescents and adults.
Percentage of Prolonged Cough Illnesses in Adolescents and Adults Caused by B pertussis Infection
Since 1987 there have been 13 published studies of adolescents and adults with prolonged cough illnesses in which the rates of B pertussis infections could be evaluated.3,1728 The breakthrough that allowed these investigations was the development of quantitative ELISA to determine antibodies to B pertussis and selected B pertussis antigens.29 The ELISA, as developed by Manclark et al,29 allows a high level of precision so that twofold changes in ELISA antibodies between acute-phase and convalescent-phase sera are diagnostic. Furthermore, B pertussis infection in adolescents and adults can be reliably diagnosed by high single-serum ELISA antibody titers.3,30 This is an important breakthrough, because adolescents and adults often do not come to medical attention until the 3rd or 4th week of illness, a time at which the peak antibody titer has already occurred.3,19 Because of this, titer changes between acute-phase and convalescent-phase sera frequently do not occur.
Presented in Table 1 are the details and results of the 13 studies in which the percentage of prolonged cough illnesses resulting from B pertussis infection are presented. As noted, the rates vary from 12% to 52%. The extreme variation in rates can be explained by multiple factors such as the number of B pertussis antigens for which antibody was determined, the biases of the study physicians, and the precision of the ELISA studies.
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The other studies in Table 1 (Seattle, WA; New South Wales, Australia; Los Angeles, CA; Nashville, TN; San Diego, CA; San Francisco, CA; Minneapolis/St Paul, MN; and Korea) were investigations of cough illnesses in populations during times at which pertussis was not recognized as an outbreak. As seen in Table 1, the determined rate in these 8 studies still varies from 12% to 50%. These differences can be explained largely by the number of antigens for which antibody was determined. In general, the studies in which antibodies to FHA, FHA and PRN, or FHA, PRN, and FIM were analyzed in addition to antibody to PT found a greater percentage of positives.
It is now recognized that infection with Bordetella parapertussis commonly elicits an antibody response to B pertussis FHA and PRN and also occasionally to B pertussis FIM.1,13 It is likely that human infection with Bordetella bronchiseptica and perhaps Bordetella holmesii also elicit similar antibody responses. In addition, it is clear that other infectious agents have proteins that, with infection, result in antibodies to FHA.27 Therefore, the studies shown in Table 1 in which FHA, PRN, and FIM antibodies contributed to the percentage of positives overestimate B pertussis infections.
In contrast with the above, antibody to PT is B pertussis specific.1 Therefore, the use of PT alone in ELISA-antibody studies gives a better indication of the rate of prolonged cough illnesses that are caused by B pertussis infection.
In Table 2 the data from the 7 studies done at nonoutbreak periods in which antibody to only PT was determined are presented. The rate in these 7 analyses varied from 1% to 17% (median: 13%). The 1% value in the San Diego study is difficult to explain, because there was a high rate of positives noted by FIM analysis.23 It seems most likely to me that the low percentage of PT antibody was an artifact caused by the care of the sera before assay.17,31
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13% of prolonged cough illnesses in adolescents and adults are a result of B pertussis infection. However, because all persons infected with B pertussis do not have an antibody response to PT, this percentage is an underestimate. In children, as many as 24% of those with culture-proven pertussis do not have a PT-antibody response.13 In adults, the percentage is
10% (ref 26; unpublished data). It is important to note that all 13 studies of adolescents and adults with prolonged cough illnesses have found evidence of B pertussis infection. These studies have been conducted in 6 countries and 7 geographic areas of the United States over a 16-year period. These data suggest that B pertussis infection in adolescents and adults is endemic and independent of the cyclic pattern observed with reported pertussis.
Rate of B pertussis Infections in Adolescents and Adults
There have been 5 studies performed in the United States in which data on rates of B pertussis infection have been presented3236 (Table 3). In all of these studies the rates have been determined by the demonstration of significant ELISA antibody-titer rises in sera collected from the same individuals over an extended time period. In 4 of the studies the sera were collected for other purposes, but in 1 study the sera were collected from controls in an adolescent and adult acellular pertussis vaccine efficacy trial (APERT). As can be seen in Table 3, the rates vary from
1% to 8% (median: 2.2%).
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The data from the APERT, Minneapolis/St Paul, and Cleveland studies indicate a total yearly adolescent and adult pertussis burden of between
800 000 and 3.3 million in the United States.25,34,36
| DISCUSSION |
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Data presently exist that suggest that vaccine-induced immunity is actually better than that induced by B pertussis infection.1,2,38 It is clear that pertussis in adults is not new, and in Germany in the early 1990s when pertussis was still epidemic, symptomatic adults were a common source for infections in young children.19,21
The data presented here suggest that B pertussis infection is common and endemic in adolescents and adults. All 9 studies done in the United States, New South Wales, Denmark, and Korea conducted at nonoutbreak times found cases in adolescents and adults.3,17,18,20,2327 Recent serologic data in adults in 8 geographic regions of the United States note the prevalence of antibodies to 4 B pertussis proteins (PT, FHA, PRN, and FIM).39 There are few differences by either geographic area or age. These data suggest that adults in Los Angeles have the same susceptibility possibilities as those in the 7 other regions studied. Similarly, 30-year-olds have the same potential risk of infection as all other adult age groups.
Household-contact studies have indicated that infection without illness is common.40,41 These data suggest that frequent exposures maintain antibody levels in many persons at protective levels. Illness only occurs when antibody values have fallen below critical values, and then exposure occurs.
The results of this study as well as a careful literature review lead me to suggest the following conclusions:
| FOOTNOTES |
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Reprint requests to (J.D.C.) Department of Pediatrics, MDCC 22-442, David Geffen School of Medicine, UCLA, Los Angeles, CA 90095-1752. E-mail: jcherry{at}mednet.ucla.edu
This work was presented in part at a meeting of the Pediatric Academic Societies; May 36, 2003; Seattle, WA.
Conflict of interest: During the last 3 years, Dr Cherry has received honoraria from GlaxoSmithKline and Aventis (both manufacturers of diphtheria and tetanus toxoids and acellular pertussis vaccines) for talks and participation in meetings related to diphtheria and tetanus toxoids and acellular pertussis vaccines.
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