Rationale: Scar formation following bacillus Calmette-Guérin (BCG) vaccination has been associated with lower all-cause mortality; the relation between scar and mycobacteria-specific protection against tuberculosis is debated.

Objectives: To evaluate the association between BCG skin reaction and mycobacteria-specific immune responses.

Methods: Apost hocanalysis was done among 214 infants in Australia randomized to vaccination with one of three BCG vaccine strains (BCG-Denmark, BCG-Japan, or BCG-Russia) given at birth or BCG-Denmark given at 2 months of age.

Measurements and Main Results: BCG skin reaction size and characteristics 10 weeks after vaccination were related to thein vitromycobacteria-specific immune responses measured in stimulated whole blood. The size and characteristics of the skin reaction correlated positively within vitroimmune responses, even after adjusting for BCG vaccine strain and age at vaccination. Specifically, the reaction size and characteristics correlated with the proportion of mycobacteria-specific polyfunctional CD4⁺T cells after stimulation with BCG and PPD and, to a lesser extent, after stimulation withMycobacterium tuberculosisorMycobacterium ulcerans. A similar correlation was observed with concentrations of IFN-γ, IL-2, tumor necrosis factor, and IL-13 in the supernatant after stimulation with BCG, PPD, andM. tuberculosisand to some degree for the proportions of mycobacteria-specific polyfunctional CD8⁺T cells and CD107⁺cytotoxic cells.

Conclusions: BCG skin reaction correlated with the magnitude of mycobacteria-specific T-cell responses. As T-cell responses play a key role in defense against mycobacteria, the relationship between BCG scar formation and protection against tuberculosis should be revisited. This may also extend to the need for BCG revaccination in scar-negative individuals.

Clinical trial registered with www.australianclinicaltrials.gov.au/clinical-trial-registries (ACTRN12608000227392).