Analysis of the direct effect of isoniazid in 10,909 miners showed a reduced incidence of tuberculosis during treatment ( 1.10 cases per 100 person-years among miners receiving isoniazid vs. 2.91 cases per 100 person-years among controls ; adjusted rate ratio , 0.42 ; 95 % CI , 0.20 to 0.88 ; P = 0.03 ) , but there was a subsequent rapid loss of protection .