Critical immunity gaps: Waning diphtheria protection among Yemen’s displaced populations calls for urgent booster strategies

Abstract

Objectives

This study evaluates seroprevalence of diphtheria toxoid immunoglobulin G antibodies among displaced populations in Abyan Governorate, which is to the best of our knowledge the first such assessment in Yemen’s conflict setting.

Methods

A cross-sectional study in 390 displaced individuals (aged 1 month–90 years) assessed anti-diphtheria antibody levels through enzyme-linked immunosorbent assay. 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 (<0.01 IU/ml). GMTs decreased significantly with age (1.04 IU/ml in those aged ≤10 years vs 0.78 IU/ml in those aged >40 years; P <0.05). Alarmingly, 63.8% of adults aged ≥40 years lacked protection. Although 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). Individuals vaccinated exhibited better protection (56.0% vs 35.5%, P <0.05), yet 15.3% remained susceptible, suggesting cold-chain or dosing failures.

Conclusions

Yemen’s displaced populations face critical immunity gaps, particularly among older adults. Suboptimal vaccine effectiveness and decreasing 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.