Geographic spillover of antimicrobial resistance from mass distribution of azithromycin.

Large-scale, placebo-controlled, cluster-randomized trials in high-mortality settings in sub-Saharan Africa demonstrated a 14-18% reduction in childhood mortality following twice-annual mass drug administration (MDA) of azithromycin among children aged 1-59 months. Azithromycin MDA also selected for antimicrobial resistance (AMR), particularly macrolide resistance. It is unknown whether the AMR from azithromycin MDA could spill over to neighboring untreated populations. If present, such geographic spillover effects could lead trials to underestimate AMR risks. We assess between-village geographic spillover effects of genotypic macrolide resistance using metagenomic deep sequencing in rectal swabs collected from 300 children in 30 monitoring villages in Niger after two years of MDA in 594 surrounding villages. Conditional permutation tests assess associations between proximal azithromycin treatment intensity and resistance gene abundance. We find no evidence of geographic spillover of macrolide resistance in untreated villages, as the genetic load of AMR remains at baseline levels in placebo-treated villages regardless of surrounding azithromycin treatment intensity (Spearman ρ = -0.05, P = 0.83). Sensitivity analyses confirm robustness across metrics, and no spillover effects are detected for other antibiotic classes. Azithromycin MDA-induced macrolide resistance appears localized to treated villages, mitigating some concerns about geographic spillover of AMR to nearby untreated villages at 24 months.