BACKGROUND: The Bacillus Calmette-Guérin vaccine (BCG) against tuberculosis is administered intradermally, and vaccination is often followed by a scar at the injection site. Among BCG-vaccinated individuals, having a scar has been associated with lower mortality. We aimed to examine the impact of vaccination technique for scarring in a high income setting, by assessing the associations between the post injection reaction, the wheal size, and the probability of developing a scar, and scar size.

METHODS: This study was nested within a clinical multicenter study randomizing 4262 infants to either BCG vaccination (BCG 1331 SSI) or no intervention. In this substudy, including 492 vaccinated infants, the immediate post BCG vaccination reaction was registered as either wheal (a raised, blanched papule at the injection site), bulge (a palpable element at the injection site), or no reaction. The presence or absence of a BCG scar and the size the scar was measured at 13 months of age.

RESULTS: Of 492 infants included, 87% had a wheal after vaccination, 11% had a bulge, and 2% had no reaction. The mean wheal size was 3.8 mm (95% confidence interval 3.7-3.9). Overall, 95% (442/466, 26 lost to follow-up) of BCG-vaccinated infants had a scar at 13 months of age. In infants with a wheal, the probability of developing a scar was 96%, declining to 87% in the case of a bulge, and to 56% in the case of no reaction (p for same probability = 0.03). Wheal size was positively correlated with the probability of getting a scar and scar size.

CONCLUSION: Scarring after BCG vaccination has been associated with lower infant mortality. In a high-income setting, we found that correct injection technique is highly important for the development of a BCG scar and that registration of the category of BCG skin reaction (as wheal, bulge, or no reaction) may be used to identify infants at risk of scar failure. Finally, the wheal size was positively associated with both the probability of getting a scar and scar size.

TRIAL REGISTRATION: The study was registered at www.ClinicalTrials.gov with trial registration number NCT01694108 .