Reason why so many people have the same scar and what causes it as photos spark confusion
People have been left scratching their heads after discovering that they all have the same kind of scar.
We're not talking a Harry Potter-esque thunderbolt across their foreheads though, but a more discreet scar that kind of looks like a tiny bite mark.
Somebody penned on Twitter alongside a photo of the unusual scar: "Can anyone explain why everyone has this mark?"
The mark is question is from a Bacille Calmette-Guérin BCG vaccine (that protects against TB). It may also be a scar from a smallpox vaccine, but this disease was eliminated back in 1980 so it's only the older generation who will have been protected against it.
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Routine smallpox vaccinations were stopped in the UK in 1971. America followed suit in 1972.
BCG vaccination
There are 150 countries that have universal infant BCG vaccination programs, some examples being Russia, Greenland, China, Indonesia, and Argentina, per The BCG World Atlas.
The US doesn't have a program for the vaccination as the Centers for Disease Control and Prevention (CDC) says it's typically given to children in countries where TB is more common. As of last year, there were only three cases of TB per 100,000 population.
Americans can get the vaccine if they meet 'specific criteria', however. The CDC says: "In the United States, BCG is only considered for people who meet specific criteria and in consultation with a TB expert. Talk to your health care provider if you have questions about the vaccine."
Clearly some people have had the vaccine as many Americans, and those from outside of the States, have the funny-looking BCG scar.
The main side affect of the vaccination is the blister that forms from the injection site, which later heals to leave a scar.
The NHS explains: "The main side effect of the BCG vaccine is a blister where the injection was given.
"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 side effects of the BCG jab that are more uncommon include fever, headache, and swollen glands under the armpit on the side of the body the vaccine was given.
Smallpox vaccination
As mentioned previously, the scar might also be from a smallpox jab – but if you were born in the late 70s onwards, it's unlikely you'll have had this vaccine.
In light of photos of people's scars and the question of why some many of us have it going viral, David Abbasi, MD - Sports Medicine & Orthopedic Surgery, weighed in on the matter.
He said on his Facebook page, where he boasts 171,000 followers: "That distinct scar isn't random, it's actually from how the smallpox vaccine was given.
"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.
"That repeated puncturing triggers a stronger localized immune response, which causes visible scarring as the skin heals."
Niamh Shackleton