[Whooping cough in Denmark among children under 1 year of age during 1980-1986].

On the basis of individual notifications, reports of cases verified by culture and letters of discharge or case reports of infants under one year with whooping cough during the period 1980-1986, the distribution of the disease is described together with vaccine efficacy and the presumed sources of infection for infants in this age group. The incidence of whooping cough in infants under one year is estimated to be approximately three cases per 1,000 per annum as compared with 150 per 1,000 at the beginning of this century and 60 per 1,000 in the nineteen fifties before routine vaccination was introduced in 1961. The first vaccination is calculated to provide a vaccine efficacy of 39%, after the second vaccination the vaccine efficacy rises to 86% and after the third vaccination the vaccine efficacy is about 98% during the remainder of the first year of life. Even although the times of vaccination were advanced in 1970, this has not reduced the frequency of whooping cough in infants as the vaccination programme has simultaneously been weakened because children are now vaccinated only three times with a weaker vaccine and the immunity is of briefer duration. The commonest sources of infection for infants are siblings. A lower infective risk is desirable and this could be obtained by vaccinating infants an extra time. Alterations in the system of notification in 1980 have resulted in increased under-notification of whooping cough in children under one year while, simultaneously, the notified cases have been described better.