Asthma as a Risk Factor for Invasive Pneumococcal Disease

BACKGROUND: The risk of invasive pneumococcal disease among persons with asthma is unknown. METHODS: We conducted a nested case-control study to examine the association between asthma and invasive pneumococcal disease. The study population included persons 2 to 49 years of age who were enrolled in Tennessee's Medicaid program (TennCare) for more than one year during the study period (1995 through 2002) and who resided in counties participating in a prospective laboratory-based program of surveillance for invasive pneumococcal disease. For each subject with invasive pneumococcal disease, 10 age-matched controls without invasive pneumococcal disease were randomly selected from the same population. TennCare files were queried to identify the presence of coexisting conditions that confer a high risk of pneumococcal disease. For the purpose of our study, asthma was defined by documentation of one or more inpatient or emergency-department diagnoses of asthma, two outpatient diagnoses, or the use of asthma-related medications. High-risk asthma was defined as asthma requiring admission to a hospital or a visit to an emergency department, the use of rescue therapy or long-term use of oral corticosteroids, or the dispensing of three or more prescriptions for beta-agonists within the year before enrollment in the study. RESULTS: A total of 635 persons with invasive pneumococcal disease and 6350 controls were identified, of whom 114 (18.0 percent) and 516 (8.1 percent), respectively, had asthma. Persons with asthma had an increased risk of invasive pneumococcal disease (adjusted odds ratio, 2.4; 95 percent confidence interval, 1.9 to 3.1) as compared with controls. Among those without coexisting conditions, the annual incidence of invasive pneumococcal disease was 4.2 episodes per 10,000 persons with high-risk asthma and 2.3 episodes per 10,000 persons with low-risk asthma, as compared with 1.2 episodes per 10,000 persons without asthma. CONCLUSIONS: Asthma is an independent risk factor for invasive pneumococcal disease. The risk among persons with asthma was at least double that among controls.