Vaccine Scars: BCG & Smallpox

Have you ever noticed a circular scar on a patient’s upper arm? Depending on the patient’s age and where they were born, the scar may be the result of either the smallpox vaccine (generally not administered since the 1970s) or the BCG vaccine (the most frequently administered vaccine worldwide today). 

Scarring from the smallpox vaccine resulted from its unique method of administration, known as the multiple puncture technique. To deliver the vaccine, a clinician would dip a bifurcated needle into the vaccine solution and then prick the skin 15-20 times in a small circle until a drop of blood appeared, leading to a pustular lesion and eventual scar formation. 

  • The JYNNEOS vaccine for mpox, an orthopoxvirus of the same genus as smallpox, is administered as a single shot but has not been associated with scar formation; however, any vaccine administered intradermally can potentially trigger keloid formation in patients with a history of keloids, for whom subcutaneous vaccine administration is preferred (JYNNEOS is available in both forms)

Interestingly, despite the common worldwide use of the BCG vaccine, the mechanism underlying its telltale scar is not well understood. Scar formation occurs in the majority of people who receive it, and the presence and size of the scar have been shown to correlate with the magnitude of the immune response to BCG vaccination.

In general, a scar from the BCG vaccine tends to have a raised center with rounded edges whereas the smallpox vaccine scar is depressed with lines radiating outward to the edge.

Source: Vaxopedia
Source: Google Images

Prior vaccination with BCG may cause a false positive reaction to a tuberculin skin test (TST), but depends primarily on the patient’s age at vaccination. BCG vaccination given in the first year of life may produce low-level TST reactivity in adults, but such reactions are typically <10 mm and rarely persist past the age of 10yo. Vaccination after the first year of life may cause a stronger and longer-lasting effect on TST; as many as 20% of individuals remain TST positive ≥10 years after vaccination at this age. Interferon gamma release assays (IGRAs) are not affected by prior BCG vaccination and are the preferred testing method in this population.

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