Hydroxychloroquine as Prophylactic Adjunct to BCG Vaccination in High-Risk Neonates: A Hypothesis for Triple Efficacy in Tuberculosis Prevention, BCG-Complication Mitigation, and SCID Safety

Background: BCG vaccination at birth remains the cornerstone of neonatal tuberculosis (TB) protection in high-burden countries. However, BCG induces severe, potentially fatal disseminated infection (BCGosis) in infants with Severe Combined Immunodeficiency (SCID). Saudi Arabia's 2019 policy delay of BCG to six months of age—intended to reduce BCGitis in SCID—leaves the vast immunocompetent neonatal population unprotected against TB during their most vulnerable window. Hypothesis: We propose that prophylactic hydroxychloroquine (HCQ), administered concurrently with BCG at 28 days of life in high-risk neonates, can achieve triple efficacy: (1) TB protection through maintenance of early BCG immunization, (2) BCG-complication mitigation through HCQ’s containment mechanisms, and (3) extended safety window for SCID diagnosis prior to irreversible BCGosis. The biological rationale rests on three pillars: HCQ’s direct intraphagosomal antimycobacterial activity, its anti-inflammatory attenuation of BCGitis, and its established safety profile via transplacental maternal administration. Critically, it is BCG itself—not HCQ—that acts as the trained immunity inducer, epigenetically reprogramming innate immune cells (monocytes, NK cells, macrophages) via H3K4me3 and H3K27ac histone modifications to mount enhanced responses to subsequent mycobacterial challenge. Evidence Base: Bernatowska et al. (2022) demonstrated that BCGosis in SCID occurs exclusively in the NK⁻ phenotype, implicating IFN-γ deficiency as the decisive pathogenic mechanism. HCQ is documented to alkalinize the phagolysosomal compartment, directly impairing mycobacterial replication, and to exert anti-inflammatory effects via TNF-α and IL-1β suppression. BCG, by contrast, is the established trained immunity inducer—epigenetically reprogramming monocytes and NK cells via mTOR-dependent pathways. These distinct and complementary roles create a biologically coherent basis for a BCG + HCQ prophylactic adjunct strategy. Conclusion: The HCQ + BCG approach, if validated, offers a pragmatic, cost-effective intervention for countries combining high consanguinity rates with significant TB burden. We call for a prospective, randomized controlled pilot trial in high-risk neonatal populations to evaluate this triple-efficacy hypothesis before broader implementation.