Why BCG Vaccine Leaves a Scar
BCG vaccination causes scarring because the normal immune response to the live attenuated mycobacterium involves formation of a bluish-red pustule within 2-3 weeks, which ulcerates at approximately 6 weeks (creating a lesion ~5mm in diameter), then forms a scab that heals with permanent scar formation within 3 months. 1
Mechanism of Scar Formation
The scarring process represents the expected local inflammatory response to the percutaneously administered live vaccine:
- Initial pustule formation occurs within 2-3 weeks as the immune system responds to the mycobacterial antigens at the injection site 1
- Ulceration phase develops around 6 weeks post-vaccination, creating a draining lesion that should be kept clean and bandaged 1
- Healing and scar formation typically completes within 3 months, leaving a permanent mark at the puncture site in the deltoid area 1
The scar represents successful local tissue inflammation and healing following the controlled mycobacterial infection at the vaccination site. 2
Variability in Scar Formation
Not all vaccinated individuals develop visible scars, with only 51-83% of vaccinated children showing permanent scarring. 3
Key determinants of whether a scar forms include:
- BCG strain used - different strains have varying rates of scar formation 1
- Administration technique - intradermal injection produces more consistent scarring than subcutaneous injection 4
- Size of injection wheal at time of administration 5
- Age at vaccination - vaccination within the first 48 hours of life has higher rates of scar failure (approximately 10%) 6
- Co-administered vaccines and micronutrients may influence local immune response 5
Clinical Significance of Scarring
The presence of a BCG scar does not predict whether the vaccine provided protection against tuberculosis disease. 3
However, research suggests broader health implications:
- Children with BCG scars have better overall survival compared to vaccinated children without scars (mortality rate ratio 0.61,95% CI: 0.51-0.74), with the strongest effect in the first two years of life 5
- Scar presence correlates with tuberculin skin test reactivity - 58% of children with scars are TST-positive versus only 15% without scars 7
- Infants with BCG lesions or scars have fewer symptoms of sepsis and more localized (rather than disseminated) infections, suggesting non-tuberculosis-specific immune benefits 8
Abnormal Scar Formation
Hypertrophic scars occur in 28-33% of vaccinated persons, and keloid scars develop in approximately 2-4%. 1
These represent exaggerated healing responses rather than vaccine failure. Accelerated or more severe reactions may indicate:
- Previous M. tuberculosis infection causing an enhanced response upon vaccination 1
- Immunocompromised status, particularly HIV infection, which increases risk of prolonged reactions and complications 1, 4
Key Clinical Pitfalls
- Absence of a scar does not mean vaccination failure - up to 49% of vaccinated children may lack visible scars yet still mount appropriate cellular immune responses (88-95% positive leukocyte migration inhibition to PPD) 6
- Reactions persisting beyond 3 months warrant evaluation for complications including BCG lymphadenitis or osteitis, not dismissal as normal healing 4
- BCG vaccination does not always leave an identifiable scar, making it difficult to confirm vaccination history based on physical examination alone 1