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What is BCG?
BCG is a vaccine for TB. It is not used for TB control in the United States, but it is often given to infants and small children in other countries where TB is common. BCG vaccine offers some degree of protection for infants and young children in preventing certain deadly forms of TB, but the protection goes away over time. Not all people react to it, and it does not prevent TB disease from developing in adults.
If you were vaccinated with BCG, you may have a positive reaction to a TB skin test. This reaction may be due to the BCG vaccine itself or to a real TB infection. But your positive reaction probably means that you have TB infection if:
your skin test reaction is large (10 millimeters or greater)
you were vaccinated many years ago (because the BCG reaction gets smaller over time)
you have ever spent time with a person with infectious TB
someone in your family has had TB
you are from a country where TB disease is very common (most countries in Latin America and the Caribbean, Africa, and Asia, except for Japan)