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BACKGROUND AND OBJECTIVES: Childhood immunization may influence the development of asthma, possibly due to lack of infections or a shift in the T-helper cell type 1/T-helper cell type 2/regulatory T cells balance. We therefore investigated whether pertussis immunization in infancy is associated with asthma medication in adolescence. METHODS: After 14 years of no general pertussis vaccination, almost 82 000 Swedish children were immunized for pertussis in a vaccination trial between June 1, 1993, and June 30, 1994. In a follow-up analysis of almost 80 000 children, their data were compared with those of ~100 000 nonvaccinated children, born during a 5-month period before and a 7-month period after the vaccination trial. Data for the main outcome variable (ie, dispensed prescribed asthma medication for each individual in the cohort during 2008-2010) were obtained from the national prescription database. Multivariate regression models were used to calculate the effect size of vaccination on dispensed asthma medication (odds ratios [OR], 95% confidence intervals [Cl]). Approaches similar to intention-to-treat and per-protocol methods were used. RESULTS: The prevalence rates of various asthma medications for study patients at 15 years of age differed between 4.6% and 7.0%. The crude ORs for any asthma medication and antiinflammatory treatment in pertussis-vaccinated children after intention-to-treat analysis were 0.97 (95% Cl: 0.93-1.00) and 0.94 (95% Cl: 0.90-0.98), respectively. Corresponding adjusted ORs were 0.99 (95% Cl: 0.95-1.03) and 0.97 (95% Cl: 0.92-1.01). Similar ORs were found after per-protocol analysis. CONCLUSIONS: Pertussis immunization in infancy does not increase the risk of asthma medication use in adolescents. Our study presents evidence that pertussis immunization in early childhood can be considered safe with respect to long-term development of asthma. [ABSTRACT FROM AUTHOR]