Smallpox is a viral, infectious disease that causes a significant skin rash and fever. During the most significant smallpox outbreaks in the 20th century, an estimated 3 out of 10 people died from the virus while many others were left disfigured, according to the Centers for Disease Control and Prevention.

Fortunately, researchers were able to create a vaccine against this virus. The virus injected is a live virus, but it isn’t the variola virus known to cause smallpox. Instead, the vaccinia virus is injected. Because this virus is so similar to the variola virus, the body can usually make enough antibodies to fight off the smallpox virus.

Through the widespread administration of the smallpox vaccine, doctors declared the smallpox virus “extinct” in the United States in 1952. In 1972, smallpox vaccines stopped being a part of routine vaccinations in the United States.

The creation of a smallpox vaccine was a major medical achievement. But the vaccine left behind a distinctive mark or scar.

While most people who have the smallpox vaccine scar are older, the U.S. Department of Health and Human Services did administer the vaccine after 1972 to healthcare workers and smallpox response teams from health departments due to the fear the smallpox virus could be used as a biological weapon by terrorists.

The smallpox vaccine is delivered in a unique manner compared to many other vaccines used today. For example, a flu shot is delivered in a one-time stick using a single needle point that goes through several layers of skin and into the muscle. The smallpox vaccine is given using a special bifurcated (two-prong) needle. Instead of puncturing the skin one time, the person administering the vaccine will make multiple punctures in the skin to deliver the virus to the skin’s dermis, which is the layer just below the epidermis that is visible to the world. The vaccine doesn’t penetrate to the deeper skin layers, such as the subcutaneous tissue.

When the virus reaches this dermal layer, it starts to multiply. This causes a small, round bump known as a papule to develop. The papule then develops into a vesicle, which looks like a fluid-filled blister. Ultimately, this blistered area will scab over. While this signals what doctors usually regard as a successful vaccination, it can leave a mark for some people.

Scars like the smallpox vaccine scar form due to the body’s natural healing process. When the skin is injured (like it is with the smallpox vaccine), the body rapidly responds to repair the tissue. The result is a scar, which is still skin tissue, but the skin fibers are arranged in a single direction instead of various directions like the rest of the skin. Normal skin cells take time to grow while scar tissue can grow more quickly. While the result is protective, people can be left with a visible reminder of skin injury.

For most people, the smallpox scar is a small, round scar that’s lower than the skin around it. Most people’s scars are no bigger than the size of a pencil eraser, although others may have larger scars. Sometimes they can be itchy and the skin feels tighter around them. This is a natural result of scar tissue development.

Some people have a different inflammatory response to skin injury. They may be prone to forming excess scar tissue in the form of a keloid. This is a raised scar that grows in response to skin injury. They are known to form on the shoulder and can cause a raised, spread scar that looks like something has spilled on the skin and hardened. Doctors don’t know why some people get keloids and others don’t. They do know those with a family history of keloids (ages 10 to 30), and those of African, Asian, or Hispanic descent are more likely to have keloids, according to the American Academy of Dermatology.

During the height of smallpox concerns, having a visible smallpox vaccine scar was a beneficial sign because health officials could assume a person was vaccinated against the virus. For example, immigration officials on Ellis Island in New York were known to inspect immigrants’ arms for the presence of the smallpox vaccine before they could be admitted to the United States.

Despite scar formation, the vaccine is known for causing fewer adverse reactions when given on the arm, compared to the buttocks or other areas.

In addition to known scarring from the smallpox vaccine, there is another vaccine that causes a similar scar. This is known as the Bacillus Calmette-Guérin or BCG vaccine. This vaccine is used to protect people against human tuberculosis. Both vaccine types can leave upper arm scars.

Often, a person can tell the difference between the smallpox vaccine and BCG scars by taking into account the following considerations:

  • The smallpox vaccine was not widely distributed in the United States after 1972. If a person was born after this time, their vaccine scar is likely a BCG scar.
  • The BCG vaccination isn’t often used in the United States, as tuberculosis occurs at low rates. However, the vaccine is used more frequently in countries where higher TB rates occur, such as Mexico.
  • Although scarring types can vary, a BCG scar tends to be raised and slightly rounded. A smallpox scar tends to be depressed, or below the skin. It’s slightly rounded, with jagged edges.

The BCG injection is also delivered intradermally, just like the smallpox vaccine.

The treatments for a smallpox scar are similar to those for scarring in general. Some tips to reduce the scar’s appearance include:

  • Wearing sunscreen at all times over the scar. Sun exposure can cause scar tissue to appear darker and thicken. This can make a smallpox vaccine appear more pronounced.
  • Applying skin-softening ointments that may help reduce the scar’s appearance. Examples include cocoa butter, natural oils, aloe, or ointments containing allium cepa (onion bulb) extract. However, these treatments haven’t been scientifically proven to full reduce the appearance of scars.
  • Talking to a doctor about dermabrasion, a process that works to remove the outer layers of skin to promote healing. The results of this method to treat scars are unpredictable.
  • Talking to a doctor about scar revision, a process that involves removing the affected skin and stitching the scar back together. While this does create another scar, ideally, the new scar is less noticeable.
  • Talking to a doctor about skin grafting, which replaces the scarred area with new, healthy skin. However, the skin edges around where the graft is placed may appear noticeably different.

If your smallpox scar developed into a keloid, you can apply silicone sheets (like a bandage) or gel to the keloid. This can help reduce the keloid’s size.

Of the more than 37,500 civilian workers who received the smallpox vaccine in 2003, an estimated 21 post-vaccination scars occurred, according to the journal Clinical Infectious Diseases. Of those experiencing scarring, the average time to noticing the scar was 64 days.

While smallpox scars may still exist, a person must evaluate whether their scar requires treatment to reduce its appearance. Most scars are removed or revised for cosmetic appearances, not health concerns.