Objective: To quantify the protection of previously infected persons from developing tuberculosis after intense exposure.
Setting: 6 hospitals and 22 nursing homes in which heavy tuberculosis exposure had occurred.
Measurements: Results of tuberculin skin tests before and after exposure and the development of tuberculosis among known reactors, both converters and nonconverters.
Intervention: All converters were given preventive therapy with isoniazid as soon as they could be identified. Nonconverters and previously known reactors were not treated.
Results: In 6 hospital outbreaks, largely aborted by prompt preventive therapy, 98 of 336 nonreactors (29%) showed skin test conversion, and, before therapy could be started, 19 (19% [95% CI, 12% to 29%]) had developed tuberculosis. No tuberculosis developed among the 238 nonconverters (0% [CI, 0% to 1.5%]) or the 76 known reactors who were not treated (0% [CI, 0.5% to 2%]). Tuberculosis developed in 5 of 209 known reactors (2.4% [CI, 0.8% to 5.5%]) in 22 nursing homes with heavy exposure, little more than 10 of 921 known reactors (1.1% [CI, 0.5% to 2%]) in 76 homes where there was no exposure (P = 0.17).
Conclusions: Healthy persons who remain nonreactive to tuberculin after heavy exposure have escaped infection and require no chemotherapy. However, if exposure is discovered immediately, it is wise to start preventive therapy in particularly heavily exposed nonreactors and discontinue it if the skin test result is still negative at 3 months. Persons who react after exposure fall into three groups: 1) converters, in whom the risk for tuberculosis warrants preventive chemotherapy, regardless of age; 2) reactors with no preexposure test results, who should be treated as converters; and 3) previously known reactors, in whom the risk for tuberculosis generally is too slight to warrant therapy. However, those who are younger than age 35 years, have human immunodeficiency virus infection, are receiving cancer chemotherapy or long-term corticosteroid therapy, or are otherwise immunocompromised should be considered for preventive therapy, regardless of the exposure.