Tuberculosis, commonly known as TB, is a bacterial infection that primarily affects the lungs, though it can also spread to other parts of the body such as the spine, brain, or kidneys. This illness is caused by Mycobacterium tuberculosis and spreads through the air when an infected person coughs, sneezes, or speaks. While TB is treatable with medication, it can be fatal if left unaddressed. A vaccine for TB exists, but its use is not universal, especially for adults in countries with a low prevalence of the disease, like the United States.
The BCG Vaccine
The vaccine used against tuberculosis is known as Bacille Calmette-Guérin, or BCG, named after its developers Albert Calmette and Camille Guérin. This vaccine is a live-attenuated preparation derived from a strain of Mycobacterium bovis, a bacterium commonly found in cattle. The original strain was attenuated, leading to a weakened form that no longer causes disease in humans but still provides some immunity.
The BCG vaccine was first administered to humans in 1921 and has since become one of the most widely used vaccines globally, with approximately 100 million children receiving it annually. Its primary global application involves the routine vaccination of infants in countries with high rates of TB. This widespread use aims to protect young children from severe forms of the disease, such as tuberculous meningitis and miliary TB, which can be particularly devastating in childhood.
Adult Recommendations and Eligibility
In countries with low tuberculosis prevalence, such as the United States, the BCG vaccine is not routinely recommended for adults. This is due to the relatively low risk of infection with Mycobacterium tuberculosis in these areas and the vaccine’s variable performance against adult pulmonary TB. Consequently, its consideration for adults is limited to specific individuals who meet particular criteria, often following consultation with a TB expert.
Certain adult groups may be considered for BCG vaccination if they face a heightened risk of exposure to tuberculosis. This includes healthcare workers in settings with a high percentage of drug-resistant TB patients, particularly when transmission to staff continues despite infection-control measures.
Individuals with continuous, unavoidable exposure to persons with untreated or ineffectively treated TB disease, especially those with drug-resistant strains, might also be candidates. This applies when separation from the infected individual is not possible or when long-term preventive treatment for the exposed person is not feasible.
The BCG vaccine is not suitable for everyone, and certain contraindications apply. It should not be administered to individuals who are immunosuppressed, including those with HIV, candidates for organ transplants, or people receiving immunosuppressive therapies. As a live-attenuated vaccine, it carries a risk of causing disseminated BCG disease in individuals with compromised immune systems. Additionally, pregnant women should not receive the BCG vaccine, as there is insufficient evidence to fully confirm its safety during pregnancy.
Efficacy and Limitations in Adults
The effectiveness of the BCG vaccine in the adult population is notably variable. While it offers considerable protection against severe forms of tuberculosis in infants and young children, its ability to reliably prevent pulmonary, or lung-based, infection in adults is limited. The protective effects of the vaccine also tend to lessen over time following vaccination.
A consideration following BCG vaccination is its impact on tuberculosis diagnostic tests. A history of BCG vaccination can lead to a false-positive result on the tuberculin skin test (TST), making it difficult to differentiate between a reaction caused by the vaccine and a true TB infection.
To address this challenge, blood tests known as interferon-gamma release assays (IGRAs) are often the preferred screening method for individuals with a history of BCG vaccination. Unlike the tuberculin skin test, IGRA results are not affected by prior BCG vaccination, providing a more accurate assessment of Mycobacterium tuberculosis infection in vaccinated individuals.
Administration and Potential Side Effects
The BCG vaccine is administered through an intradermal injection, meaning it is given into the superficial layer of the skin, typically on the upper arm. Following the injection, a characteristic local reaction usually develops over one to two months. This reaction often includes redness, mild pain, and localized swelling.
A small blister or papule may form, which can sometimes develop into a shallow ulcer. This ulcer typically heals on its own, leaving a small, permanent scar. While these local reactions are common, exaggerated responses such as larger ulcers or abscesses can occur if the injection is administered too deeply.
While generally well-tolerated, rare but more serious potential reactions can occur. One such reaction is disseminated BCG infection, often referred to as BCG-osis, which involves the spread of the vaccine strain throughout the body. This serious complication is a primary reason why the vaccine is not given to individuals with compromised immune systems. Anaphylaxis, a severe allergic reaction, has also been reported rarely.