Social media users are flooding platforms with photos of a mysterious scar they suddenly realized they share.
The mark, which resembles a tiny bite or dent, has sparked mass confusion as thousands compare their arms and demand answers. Dr. David Abbasi, MD, a sports medicine and orthopedic surgeon with 171,000 Facebook followers, stepped in last week to explain the phenomenon on his page.
“That distinct scar isn’t random, it’s actually from how the smallpox vaccine was given,” he wrote on Facebook. “Unlike modern vaccines (a quick needle poke), the smallpox vaccine used a ‘bifurcated needle’ that pricked the skin multiple times in a small area, delivering the vaccine directly into the epidermis.”
The repeated puncturing triggers a stronger localized immune response, he continued, “which causes visible scarring as the skin heals.”
For most people posting online, however, the scar likely comes from a different source entirely. The Bacille Calmette-Guérin vaccine, known as BCG, protects against tuberculosis and leaves an identical-looking mark. One Twitter user captured the viral mood by posting a photo alongside the simple question: “Can anyone explain why everyone has this mark?”
The BCG vaccine is far more common globally than many Americans realize. According to The BCG World Atlas, 150 countries maintain universal infant BCG vaccination programs, including Russia, Greenland, China, Indonesia, and Argentina. The United States does not have such a program, as the Centers for Disease Control and Prevention notes TB remains relatively rare domestically. As of last year, the U.S. Recorded only three cases per 100,000 population.
Americans can still receive the BCG vaccine if they meet specific criteria, the CDC states. “In the United States, BCG is only considered for people who meet specific criteria and in consultation with a TB expert,” the agency advises. “Talk to your health care provider if you have questions about the vaccine.”
The NHS describes exactly how the BCG scar forms. “The main side effect of the BCG vaccine is a blister where the injection was given,” the British health service explains. “This may appear as a raised bubble straight after the injection and then turn into a blister or a sore after 2 weeks. It then forms a scab and can take a few weeks or months to heal, leaving a small flat scar.”
Other BCG side effects remain uncommon but include fever, headache, and swollen glands under the armpit on the vaccinated side. The smallpox vaccine scar, meanwhile, belongs almost exclusively to older generations. Routine smallpox vaccinations ended in the United Kingdom in 1971 and in America in 1972, with the disease itself officially eliminated worldwide by 1980. Anyone born in the late 1970s onward almost certainly never received that particular jab.
The confusion between the two vaccine scars has fueled much of the online debate, with users across generations swapping theories about which shot left their mark. Photos of the distinctive scars have accumulated thousands of shares as people discover the shared physical trace of global public health campaigns most never knew they participated in.