Two vaccines are historically responsible for the round, slightly raised or indented scars that millions of people carry on their upper arms: the BCG vaccine against tuberculosis and the smallpox (vaccinia) vaccine. Both are administered into or just beneath the skin rather than deep into muscle, and both trigger a strong local immune response that heals into a permanent mark. The scar is not a side effect or a mistake. It has long been recognized as a sign that the vaccine “took,” meaning the immune system mounted the intended response.
The BCG Vaccine Scar
The BCG (Bacille Calmette-Guérin) vaccine is the most common scar-leaving vaccine still in wide use today. It protects against tuberculosis and is given routinely to newborns and infants in countries where TB remains prevalent, which includes much of Africa, South Asia, Latin America, and parts of Eastern Europe. In countries like the United States, Canada, and most of Western Europe, BCG is not part of the routine childhood schedule because TB rates are low, so people born there typically do not have the scar.
The BCG scar usually appears on the left upper arm or shoulder, though the exact site varies by country. It tends to be a small, round, slightly raised or flattened patch of skin, often lighter than the surrounding tissue. In the weeks after vaccination, infants develop a small red bump at the injection site that may blister or ooze before gradually crusting over and healing into the permanent scar over two to three months. The scar is typically less than a centimeter across, though its size and texture vary from person to person.
The Smallpox Vaccine Scar
The other well-known vaccine scar comes from the smallpox (vaccinia) vaccine, which was administered to most people born before the late 1970s or early 1980s, depending on the country. The World Health Organization declared smallpox eradicated in 1980, and routine vaccination stopped around that time. If you were born after 1980 in most parts of the world, you almost certainly did not receive it. Military personnel and certain laboratory workers are exceptions; some have been vaccinated more recently.
The smallpox scar is also found on the upper arm but looks different from the BCG mark. It tends to be larger, roughly the size of a pencil eraser or slightly bigger, and often has a slightly pitted or stippled texture. The original vaccination technique involved pricking the skin multiple times with a bifurcated (two-pronged) needle dipped in the vaccine, which produced a cluster of tiny punctures that blistered, scabbed, and healed into one distinctive mark. The scar is often more irregular in outline than a BCG scar and can appear slightly sunken rather than raised.
If you are old enough to have both a BCG scar and a smallpox scar, they are usually on the same arm but distinguishable by size and texture. The BCG scar is smaller and smoother; the smallpox scar is wider and more textured.
Why These Two Vaccines Leave Scars When Others Do Not
Most modern vaccines are injected into muscle tissue using a standard syringe and needle. They are designed to provoke an immune response without causing significant tissue damage at the injection site. You may get a sore arm for a day, but the skin heals cleanly. BCG and smallpox vaccines are fundamentally different in how they interact with the body at the point of entry.
BCG is a live attenuated vaccine, meaning it contains a weakened but still living form of a bacterium related to the one that causes TB. It is injected intradermally, right into the skin itself, where the live bacteria replicate locally and provoke an intense immune reaction. The body fights off the bacteria over weeks, and that prolonged battle in a small area of skin tissue results in enough localized damage to leave a permanent scar. The smallpox vaccine also uses a live virus (vaccinia) and is delivered into the superficial layers of the skin, producing a similar pattern of local replication, blistering, and eventual scarring.
The scar, in both cases, is considered a visible marker of successful vaccination. A 2024 review in immunopathology described scar formation following BCG or smallpox vaccination as “an established marker of successful vaccination and ‘vaccine take.'”1PubMed Central. The mark of success: The role of vaccine-induced skin scar formation for BCG and smallpox vaccine-associated clinical benefits In public health terms, being able to glance at someone’s arm and see whether they were vaccinated has historically been useful in regions where medical records are incomplete or unavailable.
What Happens When No Scar Forms
Not everyone who receives the BCG vaccine develops a visible scar, and that has been a source of concern for parents and health workers for decades. The absence of a scar does not automatically mean the vaccine failed, but it does raise questions about whether the immune response was adequate.
Several factors influence whether a scar forms. A prospective study of infants noted that scar development depends on the BCG strain used, the dose injected, and the technique of administration. Quality of the vaccine itself, proper cold-chain transport and storage, and undiagnosed immune disorders in the infant can also prevent scar formation.2PubMed Central. Scar formation and tuberculin conversion following BCG vaccination in infants: A prospective cohort study A nurse who injects the vaccine too deep, into subcutaneous fat rather than the skin itself, may produce a weaker local reaction and no visible scar. A vaccine that has been stored at the wrong temperature may contain fewer viable bacteria.
In many countries, health guidelines recommend revaccination if no scar is visible after a few months. The logic is straightforward: if the expected immune response did not produce its visible calling card, it may not have produced adequate protection either. This is not universally agreed upon, though. Some immunologists argue that the absence of a scar does not reliably predict poor immunity, since the relationship between the local skin reaction and the systemic immune response is not fully understood.
BCG Strain Matters More Than You Might Expect
BCG is not a single product. Multiple strains of the original BCG bacterium have diverged over decades of separate cultivation in laboratories around the world, and they differ in how they behave in human skin. The Danish strain, the Russian strain, the Japanese Tokyo strain, and the Brazilian Moreau strain are among the most widely used, and they do not all produce scars at the same rate or of the same size.
A study comparing the Danish and Russian strains in Guinea-Bissau found a meaningful gap: about 97% of children vaccinated with the Danish strain developed a scar, compared to 87% of those who received the Russian strain. The Danish strain also produced larger scars on average, and the difference was more pronounced in girls.3PubMed. Different effects of BCG strains – A natural experiment evaluating the impact of the Danish and the Russian BCG strains on morbidity and scar formation in Guinea-Bissau This is not just a cosmetic curiosity. Because the scar is used as a proxy for successful vaccination in many low-resource settings, a strain that produces fewer or smaller scars can complicate public health programs. Health workers may order unnecessary revaccinations, or they may misidentify children as unvaccinated when they were.
These strain differences also hint at broader biological variation. If different strains cause different levels of skin inflammation, they may also provoke different levels of immune protection. Research in this area is ongoing, and no consensus exists yet on which strain offers the best overall balance of protection and reactogenicity.
Broader Immune Effects Linked to the Scar
One of the more surprising findings in vaccine research over the past couple of decades is that both the BCG and smallpox vaccines appear to protect against more than just their target diseases. Children who receive BCG in the first days of life have lower overall mortality than would be expected from TB protection alone. The same pattern has been observed, historically, with the smallpox vaccine. Researchers describe these as “pathogen-agnostic” or “nonspecific” effects, meaning the vaccines seem to broadly prime the immune system in ways that help it fight off unrelated infections.
The scar may be a useful window into who benefits most. The 2024 review noted that both BCG and smallpox vaccines deliver “potent pathogen-specific and pathogen-agnostic effects” with “significant translational potential,” but also acknowledged that how scar formation relates to these systemic benefits “has not yet been determined.”1PubMed Central. The mark of success: The role of vaccine-induced skin scar formation for BCG and smallpox vaccine-associated clinical benefits In other words, people with more prominent scars may have stronger nonspecific immune benefits, but proving that link with certainty has been difficult. It is an active area of investigation, with implications for how BCG might be repurposed for conditions like bladder cancer (where it is already used as a treatment) or even as a general immune booster for vulnerable populations.
When the Scar Goes Wrong
For most people, the BCG or smallpox scar is a small, unremarkable mark. But in a minority, the scar can become a keloid, an overgrown, raised, sometimes painful mass of scar tissue that extends well beyond the original wound boundary. Keloids are a known complication of any skin injury, but vaccination scars are a common trigger because millions of people receive intradermal injections at a young age.
Keloids from BCG vaccination are more common in certain populations. A study of childhood BCG vaccine-induced keloids in Japan found that over 90% occurred in women, suggesting that both genetic predisposition and hormonal factors contribute. The researchers found a high percentage of allergy-related conditions and family history of keloids among affected patients, pointing toward a strong genetic component in juvenile-onset cases.4PubMed Central. Over 90% of Childhood BCG Vaccine-Induced Keloids in Japan Occur in Women More broadly, keloids are known to occur at higher rates in people with darker skin tones, and they have been documented after BCG, smallpox, and hepatitis B vaccinations.5PubMed. Extensive keloid formation and progression after each vaccination
If you have a family history of keloids or have developed keloids from previous skin injuries, it is worth mentioning this to a healthcare provider before any intradermal vaccination. Treatment options for established keloids include corticosteroid injections, silicone sheeting, laser therapy, and surgical revision, though keloids are notoriously difficult to treat and often recur.
Do Any Modern Vaccines Leave Scars?
Outside of BCG and certain smallpox-related vaccines, no vaccine in current routine use is expected to leave a permanent scar. The shift to intramuscular injections, inactivated or subunit vaccine formulations, and modern manufacturing has eliminated scarring as a normal outcome. You can receive dozens of vaccines over a lifetime and never develop a visible mark from any of them other than BCG.
That said, rare complications from any injection, such as a localized abscess, allergic skin reaction, or bacterial infection at the injection site, can occasionally produce a small scar. These are not features of the vaccine itself but of individual wound healing or procedural issues. They are uncommon and unpredictable, and they bear no resemblance to the characteristic round BCG or pitted smallpox scars.
For smallpox-related protection in the modern era, newer vaccines have been developed that do not use the traditional scarification technique. Modified vaccinia Ankara (MVA) based vaccines, for example, are administered by standard injection and do not produce the classic smallpox scar. These are the formulations now used for people at risk of mpox (formerly monkeypox) and for military personnel who need smallpox protection. If you received a smallpox-type vaccine after roughly 2010, it may well have been one of these newer formulations, and you would not have developed a scar from it.
Reading the Scar on Your Arm
If you have a small, round scar on your upper arm and you are not sure what it is from, your birth year and birth country are the two most reliable clues. If you were born in a country with routine BCG vaccination and the scar is small, round, and relatively smooth, it is almost certainly a BCG mark. If you were born before about 1980 and have a wider, slightly pitted or textured scar, it is likely from the smallpox vaccine. Some people have both, particularly those born in the 1960s and 1970s in countries that administered both vaccines.
The location can help too. BCG is typically given on the upper left arm in most countries, though some administer it on the right arm or even the thigh. The smallpox vaccine was also given on the upper arm, often the left, but practices varied. If you have two distinct round marks on the same arm, comparing their texture and size can usually tell you which is which.
Vaccine Scars in Forensic Identification
An unexpected use of vaccine scars has emerged in forensic science. Because the smallpox vaccine scar is distinctive in shape and texture, and because its presence reliably narrows a person’s birth year to before roughly 1980, forensic pathologists have used it as an identifying feature when examining unidentified remains. The BCG scar can similarly indicate a person’s country of origin, since BCG vaccination policies vary widely. A body with a BCG scar was more likely born in a country where the vaccine is routine than in one where it is not.6Forensic Science, Medicine and Pathology. The utility of smallpox vaccine scars in the forensic identification process
This use is becoming less relevant over time as the generation that received routine smallpox vaccination ages, but it illustrates how a simple mark on the skin can carry decades of medical and biographical information. For BCG scars, the forensic utility will persist longer, since BCG vaccination continues in much of the world. In settings where dental records or fingerprints are unavailable, a vaccination scar is one more piece of physical evidence that can help narrow an identification.
Why the Scar Still Matters to Researchers
Despite more than a century of BCG use and two centuries since the first smallpox vaccinations, researchers still do not fully understand the biology of how these scars form or exactly how the local skin events connect to the broad immune effects that follow. The same 2024 review that highlighted the scar as a marker of vaccine success also noted that “how scar formation occurs and how local skin-based events relate to systemic effects that allow these two vaccines to deliver powerful health promoting effects has not yet been determined.”1PubMed Central. The mark of success: The role of vaccine-induced skin scar formation for BCG and smallpox vaccine-associated clinical benefits
This gap in understanding is not trivial. If the scar is genuinely linked to the broader immune benefits of these vaccines, then learning to influence scar formation could lead to better vaccine design. It might become possible to engineer vaccines that deliberately trigger the kind of local immune response that produces both a visible scar and stronger protection, or to identify in advance which patients are unlikely to respond. For now, the small round mark on your upper arm remains one of the oldest and most visible artifacts of immunization, a biological receipt that your immune system processed a challenge and came out changed on the other side.