Millions of Kenyans can attest to having a small, round mark on their upper arm from a vaccine they received as children, but few know why the mark remains years later when signs of other childhood vaccinations have long disappeared.The mark is usually associated with a vaccine administered to protect young children against tuberculosis (TB), particularly severe forms of the disease.Known as the Bacille Calmette-Guérin (BCG) vaccine, it is typically given to infants as part of routine childhood immunisation.Unlike many vaccines that are injected into muscle or beneath the skin, BCG is administered intradermally, meaning it is injected into the skin itself.The injection typically produces a small local reaction. It may initially appear as a small raised area before developing into a sore or ulcer. As the site heals, it can leave the familiar round scar that many people carry on their upper arm.The scarring is therefore part of the body's local response to the vaccine rather than an indication that something went wrong during vaccination. BCG generally leaves some form of scarring, although not everyone develops a visible mark.It is, however, worth noting that while the presence of a scar can indicate that a person received the BCG vaccine, particularly where vaccination records are unavailable, its absence does not necessarily mean that the vaccine failed or that the person received no protection.According to the World Health Organisation (WHO), some vaccinated people do not develop a visible BCG scar.Research has also found that scar formation can vary depending on factors including the BCG strain, the way the vaccine is administered, and other factors surrounding vaccination.In terms of timing of the vaccine, WHO recommends a single dose of BCG as soon as possible after birth for infants living in areas with a high TB burden. The vaccine does not prevent every form of TB or guarantee that a vaccinated person will never develop the disease. Its major value is in reducing the risk of severe TB in young children.While some have associated the mark with protection against tuberculosis, the small circular scar on the upper arm is simply a lasting result of the childhood vaccination and does not necessarily indicate the level of protection a person has against the disease.
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