Prophylactic chemotherapy, usually with isoniazid alone, is now used in situations in which known or suspected primary tuberculous infection poses the risk of clinical disease. Isoniazid can be used alone in prophylaxis because the load of tubercle bacilli in a subclinical primary lesion is small in relation to that in reactivation tuberculosis, and experience has shown that the development of subsequent clinical disease from isoniazid-resistant strains se-lected by prophylaxis can be discounted. Unfortunately, isoniazid may cause a form of hepatitis, and the risk increases progressively after 20 years of age. Its use in older sub-jects involves balancing risk against potential benefit and requires monitoring with liver function tests.
At present, the bacillus Calmette-Guérin (BCG) vaccine (named for its originators, Calmette and Guérin) is the only available vaccine. It has been used for prophylaxis of tu-berculosis in various countries since 1923; administration is usually intradermal. It is a live vaccine derived originally from a strain of M. bovis that was attenuated by repeated subculture. Since then, it has had a checkered history, with results in different controlled trials ranging from ineffectiveness to 80% protection. In most studies, however, it has substantially decreased the highly lethal miliary and meningeal forms of tuberculosis among young children. On the basis of these results, massive immunization campaigns sponsored by the World Health Organization have been organized in underdeveloped countries.
BCG is used only in tuberculin-negative subjects. Successful vaccination leads to a mi-nor local lesion, self-limiting multiplication of the organism locally and in draining lym-phatic vessels, and development of tuberculin hypersensitivity. The latter results in loss of the PPD test as a diagnostic and epidemiologic tool, and when infection rates are low, as they are now in most Western countries, this loss may offset the possible immunity pro-duced. In general, tuberculosis rates in the West have declined as rapidly in countries that have not used the BCG vaccine as in those that have adopted mass vaccination with its occasional complications. Its potential value in these countries is restricted to population groups at particular risk. Its role in developing countries remains a matter of some contention. The BCG vaccination is contraindicated for individuals in whom cell-mediated immune mechanisms are compromised, such as those infected with the HIV.