Whole-cell and acellular pertussis vaccination programs and rates of pertussis among infants and young children

Abstract

Background: The transition from a whole-cell to a 5-component acellular pertussis vaccine provided a unique opportunity to compare the effect that each type of vaccine had on the incidence of pertussis, under routine conditions, among children less than 10 years of age.

Methods: Analyses were based on passive surveillance data collected between 1995 and 2005. The incidence of pertussis by year and birth cohort was compiled according to age during the surveillance period. We determined the association between vaccine type (whole-cell, acellular or a combination of both) and the incidence of pertussis using Poisson regression analysis after controlling for age (< 1 year, 1-4 years and 5-9 years) and vaccination history (i.e., partial or complete).

Results: During 7 of the 11 years surveyed, infants (< 1 year of age) had the highest incidence of pertussis. Among children born after 1997, when acellular vaccines were introduced, the rates of pertussis were highest among infants and preschool children (1-4 years of age). Poisson regression analysis revealed that, in the group given either the whole-cell vaccine or a combination of both vaccines, the incidence of pertussis was lower among infants and preschool children than among school-aged children (5-9 years). The reverse was true in the group given only an acellular vaccine, with a higher incidence among infants and preschool children than among school-aged children.

Interpretation: These results suggest that current immunization practices may not be adequate in protecting infants and children less than 5 years of age against pertussis. Altering available acellular formulations or adopting immunization practices used in some European countries may increase the clinical effectiveness of routine pertussis vaccination programs among infants and preschool children.

Fig. 1: Age-specific incidence of pertussis (per 10 000 population) among children less than 10 years old in Saskatoon Regional Health Authority (1995–2005), by age group.

Fig. 2: Age-specific incidence of pertussis (per 10 000 population) among children less than 10 years old in Saskatoon Regional Health Authority, by birth cohort (each year represents the year in which members of the cohort were < 1 year old). ^*The absolute incidence of pertussis at age 3 years for the cohort born in 2000 (data not shown) was estimated to be 32.2 per 10 000 population.

Fig. 3: Effect of vaccine type on the incidence of pertussis among children aged < 1 year, 1–4 years and 5–9 years. Note: wP = whole-cell pertussis vaccine, aP = acellular pertussis vaccine, wP/aP = combination of both vaccines. (^*Because of a high variance estimate, the rate ratio for the wP/aP vaccine group has not been interpreted and has only been included to maintain continuity in the figure.)