who reacted only to 100 units, tuberculosis developed in 1.2 percent (406/ 33,518). Follow-up surveillance was conducted at 2.5-year intervals. The incidence in the first interval was nearly double that in ensuing follow-up periods.

Comstock reported 18-year follow-up data on a 1946 tuberculin survey and BCG vaccination program in Muscogee County, Georgia (90). Among 1,492 individuals positive for reactivity to 5 tuberculin units of PPD, 24 (1.6 percent) had developed tuberculosis. In a larger trial conducted in Muskogee County about 10 years later, Comstock and colleagues observed more than 22,000 individuals of all ages who reacted to PPD with more than 10 millimeters of induration for 20 years (91; G.W. Comstock, personal communication). Overall, 207 of the reactors (0.94 percent) developed tuberculosis. The average annual rate was 0.73 percent. Thirty-eight pecent of the cases developed during the first 5 years of observation.

Perhaps the most useful data on the occurrence of tuberculosis in tuberculin skin test reactors excluded from BCG vaccination trials comes from the report of Comstock, Livesay, and Woolpert that includes data from the Puerto Rican trial (92). Using case registers from both Puerto Rico and New York City, they traced more than 80,000 individuals with a mean follow-up of 18.9 years. They reported data by several demographic characteristics, which are presented in Table C-14. Not surprisingly, tuberculosis occurred much more frequently among urban than rural residents. It occurred more frequently in females than males and substantially more frequently in young children.