Acellular Pertussis Vaccine Efficacy Studies in Infants
Several randomized pertussis vaccine efficacy studies were conducted in Europe and Africa to compare the safety and efficacy of the acellular and the whole-cell pertussis vaccines for the prevention of laboratory-confirmed pertussis disease in infants [33–35,38–43]. Although they provided pivotal safety and efficacy data needed for the replacement of DTP vaccines by DTaP vaccines, important differences in the study design, such as the inclusion of different whole-cell vaccines as the comparator arms, the number of acellular pertussis components included in the DTaP vaccines and different case definitions, made comparisons of the many different studies problematic. A detailed summary of the data derived from these pivotal efficacy studies is presented in Table 1. As noted in the table, vaccine efficacy varied among the different acellular vaccines, among the different whole-cell vaccines, and between different whole-cell and acellular products.
Another study, not cited in Table 1, was a trial comparing the efficacy of the two-, three- and five-component acellular pertussis vaccines against a whole-cell vaccine in 82,892 infants randomized to receive the vaccines in two different schedules; at ages 3, 5 and 12 months, or ages 2, 4 and 6 months. The primary case definition was culture-confirmed B. pertussis with at least 21 consecutive days of paroxysmal cough (typical pertussis), and the secondary definition was any culture confirmed B. pertussis with or without cough (pertussis infection). Although no significant difference was found in the efficacy of the whole-cell and the five-component acellular vaccine, the three-component acellular vaccine was less effective than either whole-cell or the five-component against culture-confirmed pertussis.
Similarly, a recent Cochrane review summarized six acellular pertussis efficacy trials, including a total of 46,283 participants . The conclusions reached from that review were that multicomponent acellular vaccines, defined as vaccines containing three or more pertussis antigens, were effective in preventing confirmed pertussis infections and were associated with fewer adverse events than whole-cell pertussis vaccines for the primary series, as well as for booster doses. However, multicomponent acellular vaccines were more effective than low-efficacy whole-cell vaccines, but less effective than the highest efficacy whole-cell vaccines. The report also noted that the efficacy of acellular vaccines varied depending on the number of vaccine components. For the multicomponent vaccines, vaccine efficacy ranged between 84 and 85% for the prevention of typical whooping cough, defined as ≥21 consecutive days of paroxysmal cough with culture or serologically confirmed B. pertussis, or contact with a household member who has culture-confirmed pertussis, and from 71 to 78% for the prevention of mild disease defined as ≥7 consecutive days of cough with confirmation of B. pertussis infection by culture or appropriate serology. For the single or two component acellular pertussis vaccines, vaccine efficacy ranged between 59 and 75% for the prevention of typical whooping cough disease, and 13–54% for the prevention of mild pertussis disease.
To assess the impact of partial immunization on pertussis disease in infants, a recent Swedish study published by Nilsson et al. reported that the incidence of pertussis disease decreased from 264 per 100,000 for unvaccinated infants 3–5 months of age to 155 per 100,000 for infants with one dose of an acellular pertussis vaccine . In infants aged 5–12 months, the dose-specific incidences were 526, 95 and 24 per 100,000 for infants with zero, one and two doses of acellular vaccine, respectively. These findings indicate that even the first dose of acellular pertussis vaccine in infancy had a significant effect on the incidence of the disease.
Expert Rev Vaccines. 2012;11(11):1331-1346. © 2012 Expert Reviews Ltd.