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, 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), over the deltoid region, producing a small local papule that later ulcerates and heals, leaving a characteristic scar confirming prior vaccination.
Mechanism/protection: stimulates cell-mediated immunity (delayed-type hypersensitivity/Th1 response) against mycobacterial antigens; its protective efficacy against pulmonary tuberculosis in adults is variable and inconsistent across populations, but provides more reliable protection against severe disseminated forms of childhood TB — specifically miliary tuberculosis and TB meningitis.
Contraindications: not given to immunocompromised individuals (including symptomatic HIV) or those with active TB, since it is a live organism and can cause disseminated BCG disease (BCGosis) in a severely immunodeficient host.
Complications: local ulcer, regional (axillary) lymphadenitis, rarely abscess formation or disseminated BCG disease.
Diagnostic note: BCG vaccination can cause a false-positive Mantoux (tuberculin skin test) result; Interferon-Gamma Release Assays (IGRAs) are unaffected by BCG vaccination and offer better specificity.