In four clinical trials of the United States Public Health Service the initially tuberculin negative persons received either isoniazid or placebo.  They were followed up with repeat tuberculin testing upon completion of prophylactic treatment or placebo for one year respectively and the proportion of converters was compared.  In two trials there was moderate protection, in the other two there was no difference between verum and placebo recipients.

The true efficacy of such prophylactic treatment cannot be ascertained very easily and trials are likely to underestimate efficacy as contacts might already be infected (but have not yet converted to a positive tuberculin skin test) at commencement of treatment. 

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