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Pertussis is an acute respiratory tract infection characterized by severe coughing spasms that may end with an inspiratory whoop. The Chinese name for this illness, "the 100-day cough," attests to its prolonged course. The incidence of pertussis in Canada has decreased by 90% since the introduction of whole-cell pertussis vaccine in 1943. However, pertussis continues to be the most common of all diseases preventable by routine childhood immunization.' It remains endemic in this country, with epidemics recurring in 3- to 5-year cycles. In the 1980s the approximate number of reported pertussis cases per year ranged from 1000 to 3000. During the current decade the approximate number of cases reported annually has increased to 2700 to 10 000.1,2
The reason for the increased incidence of pertussis is unclear; a combination of factors are likely involved. Pertussis vaccine coverage is still suboptimal: the national coverage for age-appropriate immunization by the second and seventh birthdays is estimated at 83% and 75% respectively.' The whole-cell pertussis vaccine in use until recently may not have provided adequate protection against disease; thus, during outbreaks in highly immunized populations, the effectiveness of 5 doses of wholecell vaccine against culture-positive and clinical pertussis was estimated at 57% and 59% to 71% respectively.3,4 Even when immunity is achieved, through...