Large outbreak of pertussis among young children in Chicago, 1993: Investigation of potential contributing factors and estimation of vaccine effectiveness

Background. An outbreak of pertussis from July, 1993, to April, 1994, in Chicago was investigated to identify potential contributing factors. Methods. Surveillance was enhanced to identify cases. Information from a vaccination coverage survey was used to define a retrospective cohort to estimate vaccine effectiveness of three or more doses of pertussis vaccine. Results. The median age of 218 reported cases was 8 months, 46% had Hispanic surnames and cases were clustered geographically, Vaccination status was known for 173 of 191 (91%) children younger than 6 years of age. Of these 173, 90 (52%) were younger than 7 months, and 35 (20%) children at least 7 months of age had received fewer than 3 doses of pertussis vaccine. Pertussis vaccine effectiveness was 76% (95% confidence interval, 29 to 92). Conclusions. The limited ability of the current pertussis vaccination schedule to protect young infants accounted for 52% of cases, primary vaccine failure accounted for 28% of cases and failure to vaccinate children on time accounted for 20% of cases in young children, Low vaccine effectiveness did not appear to be a contributing factor.