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Public Health - Seattle & King County

Facts about Bacille Calmette Guerin (BCG) Vaccine

What is Bacille Calmette Guerin (BCG) Vaccine?

Bacille Calmette Guerin (BCG) is the most widely used vaccination in the world. BCG is made of a live, weakened strain of Mycobacterium bovis, (a cousin of Mycobacterium tuberculosis, the TB bacteria). It was developed in the 1930's and it remains the only vaccination available against tuberculosis today. It is used because it is effective in reducing the likelihood and severity of TB in infants and young children. That is especially important in areas of the world where TB is highly prevalent, and the chances of an infant or young child becoming exposed to an infectious case are high. In the United States BCG is not used, because TB is not prevalent and the chances are small that infants and young children will become exposed. Another reason BCG is not used in the United States is because it may cause a tuberculin skin test to convert from negative to positive. This can be confusing because the TB skin test is the best available test for TB infection, and widespread use of BCG would make the skin test less useful.

Information about BCG in the United States may be very different from information about BCG that is given in many other countries. The contradictory messages often cause a lot of confusion. Some of the questions and answers below may help explain why the information about BCG in this country may seem different from the information in other countries.

Why was I given BCG?

In countries with high rates of TB, BCG is often given to infants at the time of birth because it helps prevent the more serious forms of TB disease from developing in children. In some countries BCG is given to the same person several times during childhood and early adult life, in an effort to maintain an immunity to TB. Repeated vaccination increases the likelihood of causing a positive skin test but may not increase protection against TB.

Does BCG work?

Unfortunately, the positive effect of BCG in protecting infants and young children from endemic areas from the lethal forms of TB does not extend to the adult years. Thus, many people develop active tuberculosis even though they received BCG, even in multiple doses, in earlier years. Since BCG has been used so widely and for such a long time, if it were effective it is unlikely that one third of the world's population would now have TB infection and that two million people a year worldwide would die of TB.

Could my BCG cause me to have a positive TB skin test?

BCG can cause a positive skin test, especially if the vaccine was given after early infancy, if it was given several times, and if it was given within the last five to ten years. Nevertheless, since many persons who have immigrated to the U.S. are at risk of TB even though they received BCG, the recommendation in the United States is to interpret skin tests and recommend treatment regardless of whether you have had BCG. In other words, if you have a positive skin test and are from a part of the world where TB is common, you should assume that it is due to TB exposure and implies a risk of future disease and not that it's positive because of the BCG. Nevertheless, you should let your doctor know if you had a BCG, when and if you had it more than once.

Is another vaccine being developed?

Yes. BCG continues to be studied intensively around the world for ways in which its effectiveness can be increased. Research toward a more effective TB vaccine has also been initiated. However, a new TB vaccine should not be expected for at least another 20 years.