The BCG vaccine protection fades over time, but the degree of fading depends on which type of TB you're exposed to

The BCG vaccine (Bacillus Calmette-Guérin) does not provide lifelong immunity to tuberculosis. Protection is strongest in the first 10 to 15 years after vaccination, then gradually declines. After 15 years, the vaccine still offers some protection against severe forms of TB—particularly TB meningitis in children—but protection against pulmonary (lung) TB becomes less reliable. Studies show protection ranges from 0% to 80% depending on the population studied, the strain of TB circulating in that region, and individual immune response.

This is why the BCG vaccine is most commonly given to infants and young children in countries where TB is common. In the United States, where TB rates are low, the vaccine is rarely used outside specific high-risk groups. The vaccine works by training your immune system to recognize TB bacteria, but that training weakens as time passes and your immune cells turn over.

Key Takeaways

  • BCG protection is strongest in the first 10 to 15 years after vaccination and declines gradually afterward.
  • Even after protection wanes, the vaccine continues to reduce the risk of severe TB disease, especially TB meningitis in children.
  • A single BCG dose is standard; booster shots are not routinely recommended because they do not significantly extend protection.
  • People vaccinated as children may have little to no protection by adulthood, which is why TB testing and preventive treatment are important for high-risk adults.

Why protection decreases over decades

Your immune system has two main branches: one that remembers specific threats (adaptive immunity) and one that responds immediately to any invader (innate immunity). The BCG vaccine primarily strengthens adaptive immunity by creating memory cells that recognize TB bacteria. These memory cells gradually decline in number and effectiveness as you age, even without new exposure to TB.

The decline is not sudden. Studies tracking vaccinated people show a steady drop in protection over 15 to 20 years. By age 40 or 50, someone vaccinated as an infant may have minimal protection against catching TB, though some residual benefit against severe disease may remain. This is one reason why people who were vaccinated decades ago and later move to or travel to areas with high TB rates are still at risk.

Protection against different forms of TB

The BCG vaccine is more effective at preventing some types of TB than others. It offers the strongest protection against TB meningitis (TB infection of the brain and spinal cord lining) and miliary TB (TB that spreads throughout the body), especially in young children. Protection against these severe forms persists longer than protection against pulmonary TB.

Protection against pulmonary TB—the most common form—is weaker and fades faster. This is why even recently vaccinated people can still develop lung TB if exposed to someone with active disease. The vaccine reduces the risk but does not eliminate it, and that reduction shrinks as years pass.

Booster doses and why they are not standard

A booster BCG vaccination does not significantly extend protection. Studies have shown that giving a second dose years after the first provides little additional benefit. For this reason, most countries, including the United States and those following World Health Organization guidelines, recommend only a single BCG dose in infancy.

The lack of booster benefit is one reason why TB control in countries with high disease rates relies on other strategies: early detection of active TB cases, treatment of latent TB infection in high-risk people, and improved living conditions that reduce transmission. A booster shot is not an effective way to restore protection in adults who were vaccinated as children.

What happens if you were vaccinated long ago

If you received a BCG vaccine as a child and are now an adult, your protection against TB infection is likely minimal, especially if more than 15 years have passed. This does not mean you are unprotected against severe disease—some benefit may remain—but it does mean you are not protected against catching TB if exposed.

If you work in healthcare, live with someone who has TB, or travel frequently to high-TB areas, your doctor may recommend a TB skin test or blood test to check whether you have latent TB infection. If you do, preventive treatment with medications like isoniazid can reduce your risk of developing active TB disease. This is more effective than relying on vaccine protection that faded years ago.

BCG vaccination status and TB testing

People who received BCG as children often have a small scar on the upper arm where the vaccine was injected. However, the presence or absence of a scar does not reliably indicate whether you are protected now. A scar only shows that you were vaccinated at some point; it does not show how much protection remains.

For this reason, TB testing decisions should not be based on whether you have a BCG scar. If you have risk factors for TB—such as close contact with someone who has active TB, work in healthcare, or live in a high-TB region—you should have a TB test regardless of your vaccination history. The test will show whether you have latent or active TB infection, which is what actually matters for your health.

Frequently Asked Questions

Can I get another BCG vaccine if my protection has worn off?

A second BCG dose does not provide meaningful additional protection. If you are at high risk for TB exposure, TB testing and preventive medication (if latent TB is found) is more effective than revaccination. Talk with your doctor about whether testing or preventive treatment is right for your situation.

How do I know if I still have protection from my childhood BCG?

There is no simple test to measure how much BCG protection you have left. The only way to know whether you have TB infection is through a TB skin test or blood test. These tests show current infection status, not vaccine protection level.

Does BCG protect against all types of TB?

No. BCG is most effective against severe forms like TB meningitis and miliary TB, especially in children. It offers weaker and shorter-lasting protection against pulmonary TB, the most common form. Protection against all types declines over time.

If I was vaccinated 20 years ago, am I protected now?

Probably not against infection. After 15 to 20 years, protection against catching TB is minimal for most people. Some protection against severe disease may remain, but you should not rely on it. If you have TB risk factors, ask your doctor about TB testing.

Why doesn't the BCG vaccine last as long as other vaccines?

The reasons are not fully understood, but BCG protection appears to fade faster than immunity from vaccines like measles or polio. This may relate to how TB bacteria interact with the immune system or how vaccine-induced immunity naturally declines over time. Researchers continue studying ways to improve TB vaccine durability.