[Whooping cough epidemiology in Denmark prior to and after the introduction of whooping cough vaccination. Protective effect of the vaccine and herd immunity]

Abstract

The epidemiology of whooping cough in Denmark is described on the basis of the notified cases of the disease. The frequency of whooping cough has decreased to approximately one sixteenth of the previous level in children following the introduction of vaccination for whooping cough in 1961. Following change to a programme with only three vaccinations with a weaker, non-aluminium-adsorbed pure whooping cough vaccine in 1970, whooping cough became again slightly more frequent in the nineteen seventies and eighties. No definite reduction in the proportion of cases of whooping cough among infants could be detected although this was what the 1970 programme was particularly aimed at. Isolated deaths from whooping cough still occurred in the eighties, all of these among unvaccinated infants. The risk of whooping cough in an unvaccinated child is approximately one sixth of the risk prior to introduction of vaccination. In a vaccinated child, the risk, as judged from the notified cases, is one twentieth of the risk during the time prior to introduction of vaccination. In all age groups "herd immunity" is considered to have contributed considerably to the reduced incidence. The incidence in Denmark is, however, high compared with the incidence in some other industrialized countries. A vaccination programme with more numerous whooping cough vaccinations and possibly with a more immunogenic vaccine may be recommended on the basis of the strategy aimed at keeping the incidence of whooping cough, and thus the risk of exposure, as low as possible.