The effectiveness of vaccination with whole-cell pertussis vaccine by age group in Poland 1996-2001

Recurrence of pertussis in highly vaccinated populations has been observed in many countries. Two mechanisms have been proposed to explain it: a shift in incidence towards older age groups in which the protective effect of vaccination is diminished; and vaccine-induced changes in genomic and immunological characteristics of circulating strains of Bordetella pertussis, which become different from vaccine strains, thereby reducing vaccine efficacy. Marked increase in the incidence of pertussis has been observed in Poland since 1997, following a decade of stability at a low level. As previously shown, the immunization calendar in Poland does not ensure sufficient protection among children older than 9 y. The decrease in the protective effect becomes noticeable after the age of 5. In this paper we examine changes in the effectiveness of pertussis vaccination in 4 age groups during 1996-2001, using surveillance data. We find that over that period a decrease occurred in the reported effectiveness (in children aged 2 to 5 y, from 97.3% in 1996 to 73.5% in 2001 and in 6 to 9 y-olds, from 84.3%, to 68.8%). We also discuss an alternative hypothesis that the above findings are due to selection or measurement bias resulting from incomplete reporting or imperfect diagnostic procedures.