The phrase “tuberculosis shot” actually refers to two completely different medical procedures, and the confusion between them is widespread. One is the BCG vaccine, a live weakened form of a bacterium related to tuberculosis that has been given to billions of people worldwide since 1921. The other is the tuberculin skin test (often called the Mantoux test or TST), a diagnostic injection used to check whether someone has been exposed to the TB bacterium. They look similar at the moment of injection, but they serve opposite purposes, leave different marks, and go into different parts of your body.
The BCG Vaccine
BCG stands for Bacillus Calmette-Guérin, named after the two French scientists who spent over a decade weakening a strain of Mycobacterium bovis until it could no longer cause disease but could still train the immune system. It remains the only approved vaccine against tuberculosis, more than a century after its first use in humans.1Frontiers in Immunology. A century of BCG vaccination: Immune mechanisms, animal models, non-traditional routes and implications for COVID-19 The vaccine is injected into the skin of the upper arm, usually in infancy. In countries where TB is common, newborns receive it within days of birth.
BCG works well at preventing the most dangerous childhood forms of TB, particularly TB meningitis and disseminated (widespread) TB in young children. Its track record against adult pulmonary TB, however, is much less consistent. Protection rates in adult studies have ranged dramatically depending on the population and geography, from near-zero in some tropical regions to over 70 percent in others.2Frontiers in Immunology. A Perspective on the Success and Failure of BCG This variability is one reason the United States, a low-incidence country, does not include BCG in its routine immunization schedule, while most of the world does.
The Tuberculin Skin Test
The tuberculin skin test, by contrast, is purely diagnostic. A healthcare worker injects a tiny amount of purified protein derivative (PPD), a mix of proteins extracted from TB bacteria, just under the skin of the inner forearm. The injection itself is quick and produces a small raised bubble (a wheal) that fades within minutes. The real action happens over the next two to three days: if your immune system recognizes those proteins from a prior encounter with TB bacteria, it sends immune cells rushing to the injection site, creating a firm, raised area called induration.
You return 48 to 72 hours later for a reading. A clinician measures the diameter of the induration in millimeters. Despite being in use for more than a century, the TST remains the most widely used method of screening for TB infection around the world because it is inexpensive and requires no laboratory equipment.3Applied Microbiology and Biotechnology. Skin tests for the detection of Mycobacterial infections: achievements, current perspectives, and implications for other diseases The threshold for a “positive” result varies depending on your risk level: for someone with a healthy immune system and no known TB contacts, 15 mm or more is considered positive; for healthcare workers or recent immigrants from high-incidence countries, 10 mm; and for people with HIV or other immunosuppressing conditions, just 5 mm.
How the Vaccine Muddies the Test
Here is where the confusion between the two shots becomes a real clinical problem. Because the BCG vaccine primes your immune system to recognize proteins from mycobacteria, it can also trigger a response to the TST, even if you have never been infected with actual TB. The proteins in the PPD solution overlap enough with the BCG strain that a vaccinated person’s immune system sometimes reacts to the skin test as though it had encountered the real thing. This antigenic cross-reactivity reduces the specificity of the TST in BCG-vaccinated populations.4European Respiratory Review. Multidimensional determinants of BCG-induced false-positivity in tuberculin skin testing: a global meta-analysis of 242 studies
The impact depends heavily on age. In children under five who were vaccinated as infants, the BCG effect on skin test results is pronounced. One study found that using a standard 5 mm cutoff for the TST produced good sensitivity in unvaccinated children but dropped to about 63 percent specificity in BCG-vaccinated children of the same age, meaning more than a third of positive results were likely false positives driven by the vaccine rather than by true infection.5PubMed. The impact of BCG vaccination on tuberculin skin test responses in children is age dependent: evidence to be considered when screening children for tuberculosis infection In older children and adults, the vaccine’s interference fades. Many clinicians treat a positive skin test in an adult who was vaccinated as a baby the same way they would treat a positive test in an unvaccinated adult, because the BCG effect wanes considerably over the years.
This overlap creates real-world headaches. If you grew up in a country that vaccinates at birth and then move to a country like the United States that relies on skin testing for TB screening, you may test positive even though you were never infected. Awareness of this distinction is low among immigrant communities: research has found that people from high-TB-burden countries often do not know that a blood test can sidestep the BCG problem entirely.6PubMed Central. Tuberculosis skin testing, anergy and protein malnutrition in Peru
Blood Tests That Bypass the Problem
Interferon-gamma release assays (IGRAs) were developed specifically to get around BCG-related false positives. Instead of injecting proteins under the skin, a blood sample is drawn and exposed to antigens that are specific to Mycobacterium tuberculosis and absent from the BCG vaccine strain. If the blood’s immune cells produce interferon-gamma in response, the person is likely infected with TB. Because the test uses antigens the BCG strain does not share, prior vaccination does not trigger a positive result.
Studies in BCG-vaccinated populations have consistently shown that IGRAs are a better indicator of actual TB infection than the skin test.7PubMed. Avoiding the effect of BCG vaccination in detecting Mycobacterium tuberculosis infection with a blood test This advantage holds even in immunocompromised patients. In one study of people living with HIV who had been BCG-vaccinated, the skin test was significantly affected by prior vaccination while the IGRA was not, leading researchers to recommend IGRAs as the preferred screening method for that group.8PubMed. Tuberculosis skin test, but not interferon-γ-releasing assays is affected by BCG vaccination in HIV patients
IGRAs have their own limitations. They cost more, require a functioning lab, and need a blood draw rather than a simple injection. In resource-limited settings where TB is most prevalent, the skin test remains far more practical. And neither the skin test nor the IGRA can tell you whether someone has latent TB infection or active disease. That distinction requires additional workup, including imaging and sometimes sputum cultures.9Frontiers in Microbiology. Differential Diagnosis of Latent Tuberculosis Infection and Active Tuberculosis: A Key to a Successful Tuberculosis Control Strategy
Why Skin Test Readings Are Less Precise Than They Sound
A positive skin test hinges on whether the measured induration crosses a specific millimeter threshold. That sounds objective, but measuring a raised area on someone’s forearm turns out to be surprisingly inconsistent. Two common techniques exist: palpation (feeling the induration with your fingers and marking its edges) and the ballpoint-pen method (drawing a pen toward the induration from each side and stopping where resistance is felt). Research has shown that the pen method is more reproducible, but even with it, a second measurement by the same clinician could differ by roughly 3 mm in either direction.10PubMed. Reliability of tuberculin skin test measurement When a different clinician reads the same arm, the variation widens further. The palpation method performed worse, with about 38 percent more imprecision than the pen technique.
This matters most when the induration lands near a cutoff. If your reading is 9 mm and the threshold for your risk category is 10 mm, a different reader on the same day might have called it positive. Earlier work comparing the two methods found statistically significant differences between them, particularly in that 5 to 14 mm gray zone where most borderline results fall.11Archives of Internal Medicine. Reading the Tuberculin Skin Test: Who, When, and How? None of this means the skin test is useless, but it does mean that a result near the cutoff deserves follow-up rather than a firm diagnosis in either direction.
False Negatives and Who Gets Missed
Most of the conversation around skin test accuracy focuses on false positives caused by BCG. But false negatives are an equally serious problem, and they tend to affect the people most vulnerable to TB. A malnourished person’s immune system may be too suppressed to mount a reaction to the injected proteins. Research in Peru found that people with lower body protein levels were significantly less likely to test positive on the TST, even when they had been exposed to TB, suggesting that protein malnutrition can cause what immunologists call anergy, essentially making the immune system too weak to show its hand.6PubMed Central. Tuberculosis skin testing, anergy and protein malnutrition in Peru
HIV suppresses the same immune pathways the skin test relies on, which is why people with HIV use the lower 5 mm cutoff. Very young children, people on immunosuppressive medications, and those with overwhelming active TB can also fail to produce a detectable response. In short, the skin test can miss the very populations that most need to be identified.
The Scar vs. The Bump
People also confuse the two procedures because both leave marks on the skin, though the marks look quite different and appear in different locations. The BCG vaccine is given on the outer upper arm and typically produces a small raised area, then a blister, and eventually a permanent round scar. About nine in ten infants develop a visible scar within 12 weeks of vaccination, and scar formation correlates with a stronger immune conversion.12PubMed Central. Scar formation and tuberculin conversion following BCG vaccination in infants: A prospective cohort study The initial wheal at the injection site also predicts scar development: infants who formed a clear wheal at vaccination had about a 96 percent chance of developing a scar, compared with 56 percent in those who showed no immediate skin reaction.13PubMed Central. The association between Bacillus Calmette-Guérin vaccination (1331 SSI) skin reaction and subsequent scar development in infants
The tuberculin skin test, by contrast, is placed on the inner forearm and produces a temporary mark. Any induration and redness fade within a week or two. If you have a small, round, slightly raised scar on your upper arm and grew up in a country with universal BCG vaccination, that mark is almost certainly from the vaccine, not from a skin test.
Healthcare Workers and Screening Policies
The vaccine-test confusion is especially relevant for healthcare workers, who face routine TB screening as a condition of employment. In the United States, where BCG is not part of standard childhood vaccination, annual or pre-employment skin testing has been the norm. But for internationally trained healthcare workers who were vaccinated abroad, skin testing creates the same false-positive dilemma as it does for immigrants.
European policies vary widely. A review of national recommendations found that five European countries recommend BCG only for healthcare workers in high-risk settings, five recommend it for all unvaccinated healthcare workers who test negative on the Mantoux test, and four do not recommend BCG vaccination for healthcare workers at all.14PubMed Central. European policies on tuberculosis prevention in healthcare workers: Which role for BCG? A systematic review This patchwork reflects genuine uncertainty about BCG’s value in adult healthcare workers. One decision analysis found that in workplaces with documented TB transmission despite strong infection-control measures, BCG vaccination actually resulted in less illness and death than the standard strategy of annual skin testing followed by preventive medication for those who convert.15PubMed. BCG vaccination to prevent tuberculosis in health care workers: a decision analysis In practice, many U.S. institutions have shifted to IGRA blood tests for BCG-vaccinated workers to avoid the ambiguity.
BCG Beyond Tuberculosis
One of the more surprising chapters in the BCG story has nothing to do with TB diagnosis or prevention. BCG appears to “train” the innate immune system in a broad, nonspecific way. Unlike the adaptive immune memory that most vaccines produce (where the body learns to recognize one specific pathogen), BCG reprograms certain immune cells, including monocytes, macrophages, and natural killer cells, so they respond more aggressively to a range of threats. Researchers call this trained immunity, and it involves changes to how genes are read and how cells use energy.16PubMed Central. BCG-induced trained immunity: history, mechanisms and potential applications
Animal experiments have demonstrated that BCG vaccination can protect against infections unrelated to TB. In mice, systemic BCG vaccination provided meaningful protection against influenza A virus, largely through a subset of T cells that boosted the antimicrobial activity of immune cells in the lungs in a way that did not depend on recognizing any specific flu antigen.17Nature Immunology. BCG immunization induces CX3CR1hi effector memory T cells to provide cross-protection via IFN-γ-mediated trained immunity The strength of this trained immunity response varies between individuals and appears partly determined by genetic differences in interferon-gamma signaling pathways.18Scientific Reports. The impact of interferon-γ pathway on trained immunity induction by vaccination with Bacille Calmette-Guérin
The most established clinical application of BCG outside tuberculosis is in bladder cancer. Intravesical BCG, where the bacteria are instilled directly into the bladder through a catheter, has been the gold-standard treatment for non-muscle-invasive bladder cancer at high risk of recurrence for more than 30 years.19Nature Reviews Urology. Mechanisms of BCG immunotherapy and its outlook for bladder cancer The BCG bacteria attach to the bladder wall and trigger a cascade of immune responses, including direct killing of cancer cells and recruitment of immune cells that target the tumor. More recent research has also shown that BCG can directly interact with and become internalized within cancer cells, inducing apoptosis through several pathways.20PubMed. Update on the Mechanism of Action of Intravesical BCG Therapy to Treat Non-Muscle-Invasive Bladder Cancer
Supply Problems and Strain Differences
Because BCG serves double duty as both a TB vaccine for billions of children and a cancer immunotherapy, global supply has been under chronic strain. The vaccine market has experienced national stockouts across all regions over the past decade, with the problem worsening around 2014–2015 due to manufacturing disruptions.21PubMed Central. Bacillus Calmette-Guérin (BCG) vaccine: A global assessment of demand and supply balance Only two to four manufacturers provide over half the world’s BCG supply, which means a single factory shutdown can cascade into shortages across entire continents. Africa, which carries the heaviest TB burden, has been particularly hard hit: over 40 percent of African countries reported a stockout every year since 2004.22BMJ Global Health. Tracking changes in national BCG vaccination policies and practices using the BCG World Atlas
A complication that few people outside vaccinology think about: not all BCG is the same. The original strain has diverged into several sub-strains maintained by different manufacturers around the world, and emerging evidence suggests these strains may not all be equally effective. When stockouts force countries to switch suppliers, the population may end up receiving a different sub-strain than the one their national program was designed around, with potentially different levels of protection.
New Vaccines on the Horizon
BCG’s inconsistent protection against adult pulmonary TB has driven decades of effort to develop something better. As of late 2024, around 15 to 16 TB vaccine candidates were in clinical trials across multiple platforms, including whole-cell vaccines, recombinant protein vaccines, viral vector-based vaccines, and newer mRNA vaccines.23PubMed Central. Status of Tuberculosis Vaccines Development in Clinical Trials Four of these candidates were in active Phase 3 efficacy trials, the final and most decisive stage before potential approval.24PubMed. Toward tuberculosis elimination: An update on tuberculosis vaccines in clinical trials
Some of these candidates are designed to replace BCG entirely, while others are meant to boost BCG’s effects in adolescents or adults whose childhood protection has waned. The strategies span a range of approaches: some use genetically modified versions of the BCG organism itself, some deliver TB proteins on viral vectors, and at least two are using the mRNA platform that became familiar during the COVID-19 pandemic. Modeling studies suggest that meeting global TB elimination targets by 2035 will almost certainly require a new vaccine, since BCG alone cannot break the cycle of adult-to-adult transmission that sustains the epidemic.25PubMed. New tuberculosis vaccines: advances in clinical development and modelling If any of these candidates succeed, the landscape of TB prevention could shift dramatically, and the confusing relationship between the vaccine and the skin test might finally become a historical footnote rather than an everyday clinical puzzle.