Case-control study of vaccination history in relation to pertussis risk during an outbreak among school students

BACKGROUND: Between September and December 2003, an outbreak of pertussis occurred in Cass County, MO, mostly among adolescent school children. METHODS: We conducted a 1:2 matched case-control study among school children and used conditional logistic regression to evaluate risk factors for pertussis, including the total number of vaccine doses received, age at administration of each dose of vaccine and the type of vaccine (whole cell or acellular). RESULTS: Of all 127 pertussis cases reported in this outbreak, the majority were adolescents (10-19 years of age, 50%) and adults (20 years or older, 22%); only 10% were infants and children less than 5 years of age. Because the focus of our investigation was on school-aged children, we enrolled 237 students (79 cases and 158 controls) in our study. Students missing at least one dose of the vaccine had higher risk for pertussis than those who received all 5 doses (odds ratio [OR], 2.36; 95% confidence interval [CI], 1.17-4.77). Early administration of the fifth dose of the vaccine at age 4 years was significantly associated with risk for pertussis compared with vaccination at age 5 years (adjusted OR, 2.45; 95% CI, 1.16-5.16). A short time interval (<36 months) between the fourth and fifth doses of the vaccine also tended to increase the risk for pertussis, although this association was not statistically significant. The type of vaccine was not a significant risk factor. CONCLUSION: Administering all 5 doses of pertussis vaccine and the fifth dose at age 5 years with at least 36 months between the fourth and fifth doses provided the best protection against pertussis among children and adolescents in this outbreak.