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BCG Vaccine - Comprehensive Overview

What is BCG?

Bacille Calmette-Guérin (BCG) is a live attenuated bacterial vaccine derived from an attenuated strain of Mycobacterium bovis (bovine tubercle bacillus). It was developed by French bacteriologists Albert Calmette and Camille Guérin and was first given orally during 1921-1925, with the first intradermal vaccination administered in 1927.

BCG is the only widely used live bacterial vaccine in clinical practice and one of the most widely administered vaccines in the world.

Park's Textbook of Preventive and Social Medicine, p. 231
Red Book 2021 - Report of the Committee on Infectious Diseases, p. 1211

Aim

The aim of BCG vaccination is to induce a benign artificial primary infection that stimulates acquired resistance to subsequent infection with virulent tubercle bacilli, thereby reducing morbidity and mortality from primary tuberculosis among those most at risk.

The Vaccine - Composition & Types

Consists of living bacteria derived from the attenuated bovine strain of M. bovis
The WHO-recommended strain is the "Danish 1331" strain (also used in India since January 1967 at the BCG Laboratory, Guindy, Chennai)
Two types exist:
- Liquid (fresh) vaccine
- Freeze-dried vaccine - more stable, with superior keeping qualities; this is the form currently distributed worldwide

Storage: Stable for several weeks at ambient temperature; up to 1 year if refrigerated below 10°C and protected from direct light. Once reconstituted, must be used within 3 hours. Normal saline is the recommended diluent.

Dosage

Age group	Dose
Newborns (below 4 weeks)	0.05 ml
Others (up to 1 year)	0.1 mg in 0.1 ml

The reduced dose in newborns is because their skin is thin - a full intradermal dose (0.1 ml) may penetrate deeper tissue, causing local abscess formation and enlarged axillary lymph nodes.

Administration

Route: Intradermal (ID) - standard WHO procedure
Syringe: Tuberculin syringe ("Omega Microstat" syringe with 1 cm steel 26G intradermal needle)
Site: Just above the insertion of the left deltoid muscle (left arm is chosen to maintain uniformity and help surveyors verify receipt of vaccine)
Subcutaneous injection increases risk of abscess formation
Skin must be free of antiseptic/alcohol before injection (alcohol must fully evaporate first)

Age Policy

In high-prevalence countries like India: BCG is given at birth (for institutional deliveries) or at 6 weeks of age simultaneously with DPT and polio
In low-prevalence countries: Widespread BCG is not generally recommended; restricted to high-risk groups (healthcare workers, tuberculin-negative contacts of MDR-TB cases)
BCG is given only up to 1 year of age because most children acquire natural TB infection by that age, which also confers some protection

Post-Vaccination Phenomena (Normal Reaction)

After a correct intradermal injection:

2-3 weeks: A papule develops at the injection site
~5 weeks: Papule reaches 4-8 mm diameter
Papule then subsides or breaks into a shallow ulcer (usually covered with a crust)
6-12 weeks: Spontaneous healing, leaving a permanent round scar of 4-8 mm
The individual typically becomes Mantoux-positive after 8 weeks (sometimes up to 14 weeks)

Scar absence: If no scar appears, re-vaccination is NOT required.

Mechanism of Protection

BCG generates a cell-mediated immune response (Type IV / delayed hypersensitivity), stimulating tuberculin hypersensitivity and acquired immunity against M. tuberculosis. It generates T-lymphocyte responses to mycobacterial antigens.

Protective Efficacy

Duration of protection: 15 to 20 years
The first prospective controlled trial showed 80% effectiveness over 20 years
Meta-analyses confirm BCG has approximately 80% protective efficacy against meningeal and miliary TB in children
Protection against pulmonary TB in adults is variable: ranges from 0 to 80% in different parts of the world
BCG is most effective against severe disseminated forms: TB meningitis and miliary TB

Variability in protection is thought to be partly due to prior exposure to environmental mycobacteria (which may "mask" BCG's effect), differences in BCG substrains, and host genetic factors.

Red Book 2021, p. 1243; Park's Textbook, p. 231

Complications (Adverse Reactions)

Complication	Frequency
Prolonged ulceration / suppurative lymphadenitis	1-10% of vaccinations
Osteomyelitis	Rare
Disseminated BCG infection (BCGosis)	<1 per million vaccinations
Death	Extremely rare

Disseminated BCG infection is associated with severe cellular immune deficiency (e.g., primary immunodeficiency, HIV-infected infants)
Risk of complications relates to: BCG strain used, dose, age of child, method of immunization, and skill of the vaccinator

Management of local abscess: Treated by aspiration; if unsuccessful, incision with local PAS or INH powder application. Systemic INH is NOT required. No other injection should be given into the same arm for at least 6 months after BCG.

BCG and Tuberculin Skin Test (TST)

BCG immunization can produce a positive TST (Mantoux) result, which complicates interpretation
TST is still the preferred method for detecting TB infection in children under 2 years
IGRA (Interferon-Gamma Release Assay) is preferred in BCG-vaccinated individuals because IGRAs use antigens specific to M. tuberculosis complex that are not found in BCG, eliminating false-positive results

Contraindications

Immunocompromised individuals (HIV, primary immunodeficiency, malignancy, high-dose immunosuppressants)
Neonates of HIV-positive mothers (in some guidelines)
Active skin disease at the injection site

Key Points Summary

Feature	Detail
Type	Live attenuated bacterial vaccine
Organism	Mycobacterium bovis (Danish 1331 strain)
Route	Intradermal
Site	Left deltoid (above insertion)
Standard dose	0.1 ml (0.05 ml in neonates <4 weeks)
Scar appears	6-12 weeks after vaccination
Mantoux positivity	8-14 weeks post-vaccination
Protection against	TB meningitis, miliary TB (severe forms) - ~80% efficacy
Duration of protection	15-20 years
WHO recommended in	National immunization schedule at birth or 6 weeks of age

Recent evidence (2025-2026): A 2025 meta-analysis in Lancet Microbe (PMID: 39709975) examining individual participant data confirmed that primary BCG vaccination is effective against both M. tuberculosis infection and TB disease. A 2026 meta-analysis (PMID: 41689765) evaluated BCG revaccination, with findings supporting some benefit in high-burden settings. A 2025 systematic review (PMID: 41459544) characterized BCGosis complications, confirming their rarity and association with underlying immunodeficiency.

Sources: Park's Textbook of Preventive and Social Medicine | Red Book 2021 (AAP) | Goodman & Gilman's Pharmacological Basis of Therapeutics | Janeway's Immunobiology 10e

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