Persistence of pertussis in an immunized population: results of the Nova Scotia Enhanced Pertussis Surveillance Program

Pertussis, whooping cough, is a childhood infection that causes a distinct whooping-like cough that can cause vomiting. Although vaccines against pertussis have been available for many years, occasional outbreaks still occur in North America, particularly in Canada. The actual incidence may be higher because pertussis is often underdiagnosed. The Nova Scotia Enhanced Pertussis Surveillance Program in Canada set out to determine the incidence and distribution of pertussis in a vaccinated population in Halifax, Nova Scotia. In a 28-month period, 526 cases of pertussis were reported. Diagnosis was verified by laboratory tests in 168, 32 percent, of the reported cases. The highest number of cases were reported in children between two and five. The more severe cases were found in those under one year of age, most of whom required hospitalization. Three doses of the vaccine were given to 91 percent of the patients. The vaccine was determined to be effective in 45 percent of the immunized children. It is suspected that the rate of pertussis infection was underestimated. The rate of diagnosis increased from 17 to 65 percent as the methods of laboratory diagnosis improved. This improvement indicates that cases not confirmed by laboratory analysis may be left undiagnosed and these patients will not be identified as having pertussis. The effectiveness of the current vaccine seems to be low. The reasons for the increased incidence of pertussis is unclear. More studies are needed to determine whether the vaccine has failed or whether immunized individuals have lost their immunity.