BCG vaccine scar explained

The small round mark many people call a “BCG scar” is the usual healing trace left by the tuberculosis vaccine, not a sign of active disease. (cdc.gov) Bacille Calmette-Guérin, or BCG, is a vaccine used against tuberculosis, a bacterial infection that most often affects the lungs. The World Health Organization says it is one of the world’s most widely used vaccines and reaches more than 80% of newborns and infants in countries that include it in routine immunization. (who.int) The scar forms because BCG is injected into the outer layer of the skin rather than deep into muscle. National Health Service aftercare guidance says the spot often becomes a small red lump within about 2 to 8 weeks, may ooze a little fluid or pus, then scabs and heals into a small scar over weeks to months. (royalberkshire.nhs.uk) That reaction is expected enough that several health systems give patients written instructions not to squeeze the spot, cover it tightly, or put creams on it. University Hospitals Dorset says the scab usually falls off after about 8 to 12 weeks, though healing can take 3 to 6 months. (uhd.nhs.uk) The vaccine is common globally but uncommon in routine care in the United States. The Centers for Disease Control and Prevention says BCG is not generally used in the United States because tuberculosis risk is lower, protection against adult lung tuberculosis is variable, and the vaccine can cause a false-positive tuberculosis skin test. (cdc.gov) That is why a BCG scar is often more familiar in families from countries with higher tuberculosis rates than in U.S.-born adults. The Centers for Disease Control and Prevention says many people born outside the United States have received BCG, often as infants or young children. (cdc.gov) The scar also does not mean the vaccine “took” in a simple yes-or-no way or that a person is fully protected for life. The World Health Organization says BCG has a documented protective effect against severe forms of tuberculosis in children, including meningitis and disseminated tuberculosis, while the Centers for Disease Control and Prevention says its effectiveness against adult pulmonary tuberculosis is variable. (who.int) (cdc.gov) For people who have had BCG, the main practical issue in the United States is testing. The Centers for Disease Control and Prevention says blood tests for tuberculosis are preferred over the skin test in vaccinated people because BCG can affect the skin-test result. (cdc.gov) So when social posts point to a small upper-arm dent as a “vaccine scar,” they are usually describing a real and well-known mark from BCG. It is a leftover from how the skin healed after vaccination, not evidence that someone currently has tuberculosis. (nhs.uk) (cdc.gov)