Pertussis Infection in the United States: Role for Vaccination of Adolescents and Adults

Dennis A. Brooks, MD; Richard Clover, MD


J Am Board Fam Med. 2006;19(6):603-611. 

In This Article

Epidemiology and Transmission of Disease

Pertussis infection is unique to humans. There are no animal reservoirs, and the organism cannot survive for a prolonged period in the environment. Localized to the respiratory tract, the organism is transmitted primarily by aerosol droplets from an infected person to a susceptible one. The infection is highly contagious, with attack rates ranging from 50% to 100%. The highest attack rates occur among persons with exposure within 5 feet of a coughing patient.[5]

In the pre-vaccine era, pertussis was predominantly an infection of children aged 1 to 5 years, with maternal immunity providing passive protection during an infant's first year of life. At that time, an average of 175,000 US cases were reported per year (incidence of approximately 150 cases per 100,000 population).[6] The incidence of disease declined steadily over the two decades after introduction of whole-cell pertussis vaccine, reaching an all-time low of just over 1000 reported cases in 1976.[7,8] Since then, the incidence of infection has continued to rise, with almost 26,000 cases reported to the CDC in 2004 (Figure 1).[9]

Figure 1.

Reported cases of pertussis in the United States, 1976-2004.[8,9]

Pertussis is the only vaccine-preventable disease on the rise in the US and it is severely underreported. CDC estimates that, at best, one-third of cases are believed to be reported to the CDC;[10] other estimates place the reported cases at 1 in 10 to 1 in 20 of the true incidence.[11] Although pertussis incidence remains highest among young infants, rates are also on the rise in adolescents and adults and there may be significant under-reporting in these age groups, especially those with mild or atypical infection.[7] Compared with surveillance data from 1994 to 1996, the pertussis incidence rate among adolescents and adults increased 62% and 60%, respectively, from 1997 to 2000.[7]

Increased incidence of pertussis in adolescents and adults relates to waning immunity and, likely, to a combination of previous underreporting and recent improvements in reporting processes. The longer the duration since vaccination, the higher the attack rate (Figure 2).[12] Data from a seroprevalence study by Cattaneo et al showed a peak in antibody titers at 4 to 6 years of age, coinciding with DTaP booster dosing, followed by a decline, and a second peak between 13 and 17 years of age.[13] Similar findings were documented by the National Health and Nutrition Examination Survey (NHANES), which reported a protracted decline interrupted by a peak in persons aged 40 to 45 years.[14] Since no pertussis-containing vaccine had been given past the age of 6 years, these spikes in adolescents and older persons clearly represent natural pertussis exposure. These persons not only represent potential cases, but a reservoir of disease that puts those most susceptible to significant morbidity and mortality (ie, those at the extremes of age) at risk of exposure.

Figure 2.

Relationship between time since pertussis vaccination and attack rate.[12]

Pertussis has been estimated to account for up to 17% of prolonged cough illness in adults.[15,16] Among adults with cough illness, the incidence of confirmed pertussis has been estimated at 170 to 630 cases per 100,000.[17,18] The rates among adolescents were almost 2-fold higher. Data from a prospective acellular pertussis vaccine efficacy trial (APERT) extrapolate the burden of pertussis to be nearly 1 million US cases annually in persons 15 years old.[11]

Given the large disease burden in adolescents and adults estimated by these studies, the limited number of confirmed cases of pertussis (defined in Table 1 ) in older children, adolescents, and adults is striking. A large proportion of these cases may be atypical and undiagnosed. However, according to recent data from Bisgard et al,[19] in cases where the source of pertussis was identifiable, adolescents and adults were the primary source of infection for 20% and 56%, respectively, of infants with pertussis.

Contact with infected adolescents and adults is a common source of B. pertussis infection in infants and unprotected, young children. Widespread silent transmission of pertussis within families has been reported.[20] In a study of risk factors for pertussis-related hospitalizations, siblings were the most common source (53%), followed by parents (20%), other relatives (12%), neighbors (8%), and day-care contacts (3%).[21] In a case-control study, infants of adolescent mothers (aged 15 to 19 years) were 6-fold more likely to contract pertussis, compared with infants of older mothers (aged 20 to 29 years).[22]


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