The vaccination commonly referred to as the tuberculosis shot is the Bacille Calmette-Guérin (BCG) vaccine. Introduced nearly a century ago, it is the only vaccine currently available to prevent tuberculosis (TB). The BCG shot is derived from an attenuated, or weakened, strain of Mycobacterium bovis, a bacterium related to the human TB-causing agent. It works by stimulating the immune system to recognize and fight the TB bacteria, thereby reducing the risk of developing the active disease. Despite its widespread use, the vaccine’s effectiveness varies significantly across different nations.
What the BCG Vaccine Protects Against
The primary benefit of the BCG vaccine is its ability to protect the youngest population. When administered to infants and young children, it is highly effective at preventing the most severe and life-threatening forms of the disease. This includes protection against TB meningitis, an infection of the membranes surrounding the brain and spinal cord, and miliary TB, where the bacteria spread throughout the body. Studies show the vaccine reduces the risk of these severe childhood diseases by 80% or more.
Protection is notably different when considering pulmonary TB, the most common form of the illness affecting the lungs. The BCG vaccine’s effectiveness against adult pulmonary TB is highly variable, ranging widely from zero to 80%. This inconsistent protection against the contagious form of the disease is a major limitation. While BCG saves young lives by preventing severe childhood disease, it does not reliably stop the spread of infection within a community.
Global Differences in Vaccine Policy
The varying efficacy against adult pulmonary disease is a key factor influencing global vaccination policies. In countries with a high prevalence of tuberculosis, the World Health Organization recommends administering a single dose of the BCG vaccine to all newborns as part of routine immunization programs. This strategy is based on the significant risk of infants being exposed to TB and the vaccine’s proven ability to prevent severe childhood disease in high-risk settings. Over 80% of countries globally follow this universal vaccination policy for neonates.
Conversely, in nations with a low incidence of tuberculosis, such as the United States, Canada, and many Western European countries, the vaccine is generally not given routinely. When the disease burden is low, the risk-benefit calculation shifts dramatically. In these low-incidence regions, the potential for side effects and interference with TB screening tests outweigh the benefit of universal vaccination. Instead, these countries may selectively vaccinate only small groups of children at particular risk, such as those with continuous exposure to a person with untreated TB.
How the Shot Affects TB Screening Tests
The most practical concern for individuals who have received the BCG shot is its impact on tuberculosis screening tests. The traditional method is the Tuberculin Skin Test (TST), also known as the PPD test, which involves injecting a small amount of purified protein derivative under the skin. A positive reaction is indicated by an area of raised, firm swelling, suggesting the person has been exposed to the TB bacteria. However, because the BCG vaccine contains weakened mycobacteria, the immune response it creates can lead to a false positive result on the PPD test.
Since the PPD test cannot distinguish between an immune response from the vaccine and an actual infection, it becomes an unreliable tool for diagnosing latent TB infection in vaccinated individuals. A positive PPD result in a person with a history of BCG vaccination necessitates further, more specific testing to confirm whether a true infection exists.
To overcome this diagnostic challenge, modern screening often relies on Interferon Gamma Release Assays (IGRAs), such as the QuantiFERON-TB Gold test. IGRAs are blood-based tests that measure the immune system’s response to specific proteins unique to the Mycobacterium tuberculosis bacteria. Since these specific proteins are not present in the BCG vaccine strain, an IGRA test is not affected by prior BCG vaccination. For this reason, IGRAs are the preferred method for accurately screening for latent TB infection in people who have received the BCG shot.
Administration, Safety, and Side Effects
The BCG vaccine is typically administered as a single dose via an intradermal injection, meaning it is injected directly into the top layer of the skin, usually on the upper arm. The injection site undergoes a predictable series of reactions that signal a successful immune response. Initially, a small, red, raised blister may form within two to four weeks.
This blister will eventually ulcerate, form a scab, and then heal over several weeks or months. The most common and expected outcome is the formation of a small, permanent scar at the injection site. Other common, though usually mild, side effects include temporary swelling of the lymph nodes under the armpit or a low-grade fever. More serious complications, such as localized abscesses or disseminated infection, are extremely rare and occur primarily in individuals with severe underlying immune deficiencies.