Influence of isoniazid on naturally acquired tuberculin allergy and on induction of allergy by BCG vaccination.

Previous studies on the influence of isoniazid on the size of the tuberculin reaction have given conflicting results. A controlled study in an area with high prevalence of low-grade allergy has been carried out by the administration of isoniazid or placebo tablets. For those not vaccinated with BCG, isoniazid in a single daily dose of 5 mg/kg body-weight tended to reduce somewhat the size of the tuberculin reaction among those with reactions of 12 mm or more at the initial tuberculin test.In people who were vaccinated with BCG, isoniazid given simultaneously resulted in significantly less increase in the size of post-vaccination tuberculin reactions as compared with controls; the difference was still significant, in tests conducted 4(1/2) months after the discontinuation of isoniazid. However, in spite of isoniazid, the post-vaccination allergy induced by BCG was quite considerable. This considerable increase in post-vaccination allergy suggests that the vaccination was successful in spite of the administration of isoniazid and makes it clear that primary chemoprophylaxis could be combined with BCG vaccination.Administration of isoniazid for 2 months is estimated to have killed about 90% of the bacilli in the BCG vaccine injected intracutaneously.