2. BCG Vaccine Scar (Tuberculosis Vaccine)
Discover more
FAMILY
family
Family
Another very common cause of a round scar on the arm—or sometimes the shoulder—is the BCG vaccine, which protects against tuberculosis (TB).
BCG vaccine
How it creates a scar:
Unlike many injections that heal without marks, the BCG vaccine often produces a small skin reaction. After injection, a small bump may form, then turn into a blister or ulcer before healing into a permanent scar.
Discover more
family
FAMILY
Family
Appearance:
Round or oval mark
Flat or slightly sunken
Usually 2–8 mm in size
Often on the upper arm (right or left depending on country protocol)
Why does it scar?
The vaccine uses a weakened strain of bacteria that stimulates the immune system strongly at the injection site. That immune response is what creates the lasting mark.
Important note:
Not everyone develops a visible BCG scar, but many do. Its presence or absence does not always indicate how well the vaccine worked.
3. Insect Bites That Became Scarred
Sometimes, what looks like a “perfect round scar” actually started as a mosquito bite, spider bite, or other insect bite that became infected or irritated.
How it happens:
Bite becomes inflamed
Scratching breaks the skin
Bacteria enter the wound
Healing produces a circular scar
Why round?
The immune response and inflammation often spread evenly around the bite, creating a round healing pattern.
Signs it was a bite:
Itching at the beginning
History of swelling or redness
Possibly multiple similar marks
Slight discoloration (brown or lighter than skin)
These scars are completely harmless but can remain visible for years.
4. Minor Skin Infections or Boils
Another common reason is a past skin infection, such as a boil or abscess.
What happens:
A blocked hair follicle or minor infection can form a painful lump. When it heals—especially if it drains—it may leave behind a circular scar.
Appearance:
Round, sometimes slightly raised or sunken
May be darker than surrounding skin
Can feel slightly firm
Why circular?
Because infections often spread outward evenly before healing inward, they tend to leave round or oval shapes.
5. Medical Procedures or Injections