[Pertussis in 1997].

In 1997 an epidemic increase of pertussis was observed in Poland. The incidence was 5.4/100,000 population and was more than six times higher than in the preceding year. No clear reason for a sudden increase in pertussis incidence was found, in particular the vaccination coverage rates have remained high. In December 1997 the DTP vaccine coverage rate for children below two with four doses of DTP was 97.5%. The distribution of cases according to age during the last 20 years was analysed and it was shown that since the beginning of the '90 a growing proportion of cases occurs among fully vaccinated children and the average age of the cases has a steadily growing tendency. A hypothesis was put forward that the come-back of pertussis is presently due to waning of short-term immunity following immunisation, in cohorts of children who grew up in conditions of very low B. pertussis natural transmission. An open question remains whether to introduce to the immunisation calendar an additional booster dose of pertussis vaccine in school-aged children.