*3.1. General Demographics of Study Population*

Of the 32,337 available specimens from children aged 0–14 years, 31,456 (97%) specimens yielded results for tetanus and diphtheria immunity. A slightly higher percentage of 5–9-year-olds (39.5%) compared to younger (30.2%) and older (30.3%) age groups, and an almost equal proportion of males (51.0%) and females (49.0%) were tested. The highest proportion of participants tested was in the north west geopolitical zone (27.9%) compared to almost equal proportions in other zones (range: 12.7–16.0%). Rural areas accounted for a higher proportion of specimens tested (58.6%) than urban areas (41.4%) (Table 1).


**Table 1.** Demographic characteristics of children tested for tetanus and diphtheria, Nigeria, 2018.

#### *3.2. Tetanus and Diphtheria Seroprotection*

Overall, 70.9% (95% confidence interval (CI): 69.9–72.0%) of children aged <15 years had at least minimal seroprotection (≥0.01 IU/mL) against tetanus and 84.3% (95%CI: 83.6–85.0%) had at least minimal seroprotection (≥0.01 IU/mL) against diphtheria (Table 2). Full seroprotection (≥0.1 IU/mL) was 42.2% (95% CI: 41.2–43.3%) for tetanus and 41.7% (95% CI: 40.9–42.5%) for diphtheria; long-term seroprotection (≥1.0 IU/mL) was 15.1% (95% CI: 14.5–15.7%) for tetanus and 6.0% (95% CI: 5.7–6.4%) for diphtheria (Tables 2 and S1).

The proportion of children aged <15 years with seroprotection against tetanus and diphtheria increased with age (*p* < 0.001). While minimal tetanus seroprotection was not different between male and female children, full- and long-term tetanus and diphtheria seroprotection were higher in male compared to female children (Tables 2 and S1). Tetanus and diphtheria minimal, full, and long-term seroprotection levels were lowest in the north west and north east geopolitical zones, while the south east and south south geopolitical zones had the highest tetanus and diphtheria seroprotection levels (Tables 2 and S1).

Tetanus and diphtheria seroprotection varied significantly by state and age group (Figure 1). The proportion of children with minimal tetanus seroprotection among children aged 0–4 years was at least 90% in Enugu, Ebonyi, Cross-river, and Edo states in the south east and south south zones. While most of the states in the north east and north west geopolitical zones had very low seroprotection against tetanus, three states, Adamawa, Taraba, and Yobe, had higher minimal tetanus seroprotection (>70%) among children aged 0–4 years in these zones. Only three states, FCT Abuja, Edo, and Ebonyi, had >70% of children aged 0–4 years with full tetanus seroprotection; however, seven states had >70% full tetanus seroprotection among children aged 10–14 years (Figure 1b). It is noticeable that tetanus seroprotection was higher among children aged 10–14 years compared with children aged 5–9 years in some states in the north west and north east geopolitical zones despite the lack of childhood Td booster doses in the national immunization program. The proportion of children with diphtheria seroprotection increased with age across the majority of the states (Figure 1c,d). While Sokoto, Zamfara, and Jigawa in the north west zone had less than 50% of children aged 10–14 years who had minimal seroprotection against tetanus, those states had greater than 90% of children with minimal diphtheria seroprotection (Figure 1a,c).

*Vaccines* **2023**, *11*, 663

