Clinical efficacy of the Japanese acellular pertussis vaccine after intrafamiliar exposure to pertussis patients.

A major increase in the number of pertussis cases was observed subsequent to the complete discontinuation of mass-immunization against the disease in Aichi Prefecture in central Japan. The discontinuation of DTP vaccine started in 1975 and lasted for five years. Since 1978, epidemics of pertussis were observed in children mainly in the unvaccinated age groups. The outbreaks continued in the following year and subsided in 1980 when vaccination was reintroduced. Since 1981 newly developed acellular pertussis vaccines have been used for mass-immunization. The incidence of pertussis has remained low and no epidemics of the disease have been observed among children vaccinated with the new vaccines. During the epidemics in Aichi Prefecture, the protective efficacy of the new vaccines were studied in 21 vaccinees with intrafamiliar exposure to the disease. Clinical observations showed that the vaccine reduced the attack rate of pertussis after household contacts from 85% to 9.5%. It was concluded that the vaccine effectively prevented pertussis.