Determinants of subclinical leprosy among household contacts in Indonesia: serological and socio-demographic factors

Background: Leprosy remains a public health challenge in Indonesia, which ranks third globally after India and Brazil. Subclinical infection among household contacts contributes to ongoing transmission, as individuals infected with Mycobacterium leprae (M. leprae) without symptoms may serve as undetected reservoirs. This study investigated serological and sociodemographic determinants associated with subclinical M. leprae infection among household contacts of leprosy patients in Tangerang, Indonesia. Methods: A cross-sectional study was conducted in 2020 among 320 household contacts of confirmed leprosy index cases recruited through purposive sampling. Anti-Phenolic Glycolipid-1 (PGL-1) IgM antibodies were detected using an in-house enzyme-linked immunosorbent assay (ELISA). Bivariate analysis using Chi-square and t-tests assessed preliminary associations, and multivariate logistic regression was applied to identify independent predictors of seropositivity, adjusting for potential confounders. Results: Overall, 43.8% of household contacts were seropositive for anti-PGL-1 IgM antibodies. Multivariate analysis revealed that a history of Bacille Calmette-Guerin (BCG) vaccination was associated with significantly lower odds of seropositivity (adjusted OR = 0.514; 95% CI [0.291–0.907]; p = 0.018), while the presence of a visible BCG scar was associated with nearly twofold higher odds (adjusted OR = 1.953; 95% CI [1.117–3.415]; p = 0.024). No significant associations were found between sociodemographic factors such as age, sex, or contact duration, and seropositivity. Conclusion: BCG vaccination status and visible BCG scars were key determinants of anti-PGL-1 seropositivity, suggesting complex interactions between vaccination, immune response, and exposure to M. leprae. The study highlights the protective role of BCG-induced immunity while emphasizing the need for standardized scar assessment and continuous surveillance of household contacts. Although limited by its cross-sectional and purposive design, the integration of immunological and epidemiological data represents a strength, providing evidence to support Indonesia’s Zero Leprosy 2030 control strategy.