Question
BCG vaccine
Answer
BCG (Bacille Calmette-Guérin) vaccine is a live attenuated vaccine derived from Mycobacterium bovis, developed by Calmette and Guérin through prolonged serial subculture (over 230 passages on glycerinated bile-potato medium) of a virulent M. bovis strain, progressively attenuating its virulence while retaining immunogenicity.
Administration: given as a single intradermal injection, typically at birth or shortly after (part of the routine national immunization schedule in most TB-endemic countries, including India); administered over the deltoid region of the upper arm, producing a small local papule that later ulcerates and heals, leaving a characteristic scar used to confirm prior vaccination.
Mechanism/protection: stimulates cell-mediated immunity (a delayed-type hypersensitivity/Th1 response) against mycobacterial antigens; its protective efficacy against pulmonary tuberculosis in adults is variable and inconsistent across different populations/studies, but it provides more reliable and substantial protection against severe disseminated forms of childhood TB — specifically miliary tuberculosis and TB meningitis — making early-life vaccination valuable even where overall pulmonary protection is uncertain.
Contraindications: not given to immunocompromised individuals (including symptomatic HIV infection) or those with active tuberculosis, since it is a live organism and can cause disseminated BCG disease (BCGosis) in a severely immunodeficient host.
Complications: local complications can include a persistent ulcer, regional (usually axillary) lymphadenitis, and, rarely, abscess formation; disseminated BCG disease is a serious but rare complication, occurring predominantly in undiagnosed severe combined immunodeficiency or advanced HIV.
Diagnostic note: because BCG vaccination itself induces tuberculin sensitivity, it can cause a false-positive Mantoux (tuberculin skin test) result, complicating interpretation of TST in previously vaccinated individuals — Interferon-Gamma Release Assays (IGRAs) are not affected by BCG vaccination and offer better specificity in this context.