Critical Immunity Gaps: Waning Diphtheria Protection Among Yemen’s Displaced Populations Calls for Urgent Booster Strategies

• First conflict-zone serosurvey in Yemen reveals critical protection gaps in adults >40 years. • Age-dependent immunity decline. • Vaccination system failures. • Males exhibit higher long-term protection despite similar geometric mean titers. • Proposes targeted adult booster campaigns and strengthened serosurveillance in conflict zones. Objective: This study evaluates seroprevalence of diphtheria toxoid IgG antibodies among displaced populations in Abyan Governorate, the first of such assessment in Yemen’s conflict setting. Methodology: A cross-sectional study of 390 displaced individuals (1 month - 90 years) assessed anti-diphtheria antibody levels via ELISA. Geometric mean titers (GMTs) and seroprotection rates were analyzed by age, gender, and vaccination status. Results: Overall seroprotection was 76.2%, with 23.8% susceptible (40-year-olds; P<0.05). Alarmingly, 63.8% of adults ≥40 years lacked protection. While GMTs showed no gender difference (males: 1.04±0.17; females: 1.05±0.23 IU/mL), males had higher long-term protection (56.3% vs. 44.0%, p<0.05). Vaccinated individuals exhibited better protection (56.0% vs. 35.5%, p<0.05), yet 15.3% remained susceptible, suggesting cold-chain or dosing failures. Conclusion: Yemen’s displaced populations face critical immunity gaps, particularly among older adults. Suboptimal vaccine effectiveness and waning immunity underscore the need for: targeted booster campaigns for adults; strengthened vaccine delivery systems; and integrated serosurveillance in conflict zones. These findings provide evidence for revising national immunization strategies in humanitarian crises. Diphtheria, seroprevalence, displaced population, Yemen, humanitarian crisis, vaccine immunity