Objective: To determine tuberculosis transmission dynamics in San Francisco and its association with country of birth and ethnicity.
Methods: Restriction fragment length polymorphism (RFLP) typing was performed on Mycobacterium tuberculosis isolates from culture-positive pulmonary tuberculosis patients in San Francisco (1991 through 1996), using IS6110 as a probe. Patients were assigned to clusters based on mycobacterial isolates with identical DNA fingerprints. Clusters were assumed to have arisen from recent transmission. A transmission index was defined as the average number of culture-positive pulmonary tuberculosis cases generated by a single source case and calculated for subgroups.
Results: The transmission index was higher in US-born (0.59) than in foreign-born groups (0.21), and was highest in blacks, in particular those aged under 35 years. The increased transmission index among blacks was not explained by smear-positivity, human immunodeficiency virus infection, or increased susceptibility to disease progression.
Conclusion: US-born tuberculosis cases generated more secondary cases than immigrants. Young blacks appear to be a high-risk group for tuberculosis transmission. These results suggest the need to develop interventions targeted towards this risk group.