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

## TL;DR

A study in Yemen finds that older displaced adults have low diphtheria immunity, suggesting the need for booster campaigns and improved vaccine systems.

## Contribution

This is the first serosurvey in a conflict zone revealing age-dependent immunity gaps and gender differences in diphtheria protection.

## Key findings

- 63.8% of adults aged ≥40 years lacked diphtheria protection.
- Males showed higher long-term protection despite similar antibody levels.
- Vaccinated individuals still had 15.3% susceptibility, indicating vaccine delivery issues.

## Abstract

•First conflict-zone serosurvey in Yemen reveals critical protection gaps in adults aged >40 years.•Age-dependent immunity reduction.•Vaccination system failures.•Male participants exhibit higher long-term protection despite similar geometric mean titers.•Proposes targeted adult booster campaigns and strengthened serosurveillance in conflict zones.

First conflict-zone serosurvey in Yemen reveals critical protection gaps in adults aged >40 years.

Age-dependent immunity reduction.

Vaccination system failures.

Male participants exhibit higher long-term protection despite similar geometric mean titers.

Proposes targeted adult booster campaigns and strengthened serosurveillance in conflict zones.

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.

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.

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.

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.