The Bacille Calmette-Guérin (BCG) vaccine is a globally administered inoculation. Developed between 1908 and 1921 by French bacteriologists Albert Calmette and Camille Guérin, the vaccine is derived from an attenuated, or weakened, strain of Mycobacterium bovis. This bacterium is closely related to the organism that causes human tuberculosis (TB). The strain was subcultured over many years, rendering it harmless to humans while retaining the ability to stimulate an immune response. The BCG vaccine remains the only licensed vaccine specifically intended for use against tuberculosis.
The Primary Target: Tuberculosis Prevention
The vaccine’s primary function is to stimulate the immune system to recognize and fight off subsequent infections from Mycobacterium tuberculosis. Upon administration, the weakened M. bovis strain is taken up by antigen-presenting cells, such as macrophages and dendritic cells, which then process the bacterial components. This process activates both the innate and adaptive arms of the immune system, leading to the development of T cells that can produce signaling molecules like interferon-gamma (IFN-γ).
This immune activation provides significant protection, particularly against the most severe forms of tuberculosis in young children. The BCG vaccine is highly effective at preventing childhood tuberculous meningitis and disseminated, or miliary, TB, which are often fatal conditions in infants.
The protective efficacy, however, is notably inconsistent against the most common form of the disease, pulmonary TB, especially in adolescents and adults. Protection tends to wane significantly as a person ages. The variable effectiveness has been a long-standing challenge, with different clinical trials showing widely ranging results, sometimes from 0% to 80% efficacy depending on the study’s geographic location.
Rationale for Geographic Variation in Use
The BCG vaccine’s variable efficacy and other factors have led to dramatically different public health policies regarding its use across the world. In areas with a high prevalence of tuberculosis, such as many countries in Africa and Asia, the vaccine is routinely administered to all healthy newborns shortly after birth. This mass vaccination strategy aims to protect the most vulnerable population—infants—from severe, life-threatening TB.
Conversely, in countries with a low incidence of TB, including the United States and many Western European nations, routine BCG vaccination for the general population is not recommended. In these low-risk areas, vaccination is typically reserved only for targeted high-risk groups, such as children with a high likelihood of exposure who cannot receive preventive medication. The epidemiological rationale for this approach is twofold: the low disease burden means the potential benefits do not outweigh the associated costs, and the vaccine’s impact on diagnostic testing creates a significant complication.
A major public health concern is that previous BCG vaccination causes a biological reaction that complicates subsequent screening for TB infection. In low-prevalence settings, widespread vaccination would interfere with the reliable use of the Tuberculin Skin Test (TST), a standard screening tool. This interference makes it difficult to distinguish between a positive result caused by the vaccine and a true M. tuberculosis infection.
Administration, Expected Reactions, and Diagnostic Impact
The BCG vaccine is administered using an intradermal injection, meaning the vaccine is delivered just beneath the top layer of the skin, typically on the left upper arm. This precise method is necessary to ensure the immune system is stimulated correctly and helps minimize the risk of a severe local reaction. Following the injection, the body mounts a localized inflammatory response that is considered a normal reaction.
The site of injection usually develops a small red bump or blister several weeks after vaccination, which may turn into a shallow ulcer over time. This lesion typically heals naturally over a few weeks or months, leaving behind a small, flat, circular scar, often referred to as the BCG scar. This characteristic scar confirms that the vaccination was successfully administered and initiated an immune response.
The most significant practical consequence of receiving the BCG vaccine is its effect on the Tuberculin Skin Test, also known as the Mantoux test. The TST works by detecting a person’s immune reaction to certain proteins from the TB bacterium, and because the BCG vaccine contains a related live bacterium, it can cause a false-positive TST result. This interference can lead to unnecessary follow-up testing and anxiety for the patient.
To address this diagnostic challenge, healthcare providers increasingly utilize alternative testing methods for individuals who have received the vaccine. Interferon-Gamma Release Assays (IGRAs), which are blood tests, are the preferred method for screening BCG-vaccinated individuals for TB infection. These blood tests are not affected by prior BCG vaccination, allowing for a more accurate distinction between a vaccine-induced reaction and a true infection.