Epidemiology of pertussis in Denmark: the impact of herd immunity.

Abstract

An evaluation is presented of the Danish pertussis immunization programme, which consists of three injections of plain whole-cell pertussis vaccine given alone at ages 5 weeks, 9 weeks, and 10 months. The incidence of pertussis in vaccinated and unvaccinated children since the start of vaccination was obtained from the notification system for infectious diseases. Data for vaccination coverage were obtained from the National Social Security. The data for 1980-1986 were supplemented with data from culture-verified cases and hospitalized cases. Compared with other countries using four injections, incidence rates in Denmark are high, especially in pre-school years, leaving infants at a relatively high risk for contracting pertussis from siblings. However, compared with the era before general vaccination, the incidence of pertussis has fallen to one-sixteenth of its former levels. Today, only one in 20 vaccinated, and one in six unvaccinated children develop pertussis before the age of 15 years. This considerable fall, which has also occurred among unvaccinated children, is used to elucidate the importance of herd immunity, which, with the relatively high vaccination coverage in Denmark, was found to play a major role. The importance of herd immunity is stressed, and it is recommended that a fourth injection of pertussis vaccine is introduced to bring incidence rates down to the very low values found in countries with more intensive vaccination programmes.