**Table 2.** Tetanus and diphtheria seroprotection among children aged <15 years, Nigeria, 2018.

(**a**) Minimal tetanus seroprotection (ǃ0.01 IU/mL) by age group and state

(**c**) Minimal diphtheria seroprotection (ǃ0.01 IU/mL) by age group and state

(**b**) Full tetanus seroprotection (ǃ0.1 IU/mL) by age group and state

(**d**) Full diphtheria seroprotection (ǃ0.1 IU/mL) by age group and state

**Figure 1.** Tetanus and diphtheria minimal and full seroprotection among children aged <15 years by age group and state, Nigeria, 2018 (**a**–**d**).

The proportion of children <15 years with minimal or full tetanus seroprotection was higher in urban compared to rural areas (Table 2) and was statistically significantly different in all geopolitical zones (*p* < 0.05) except in the north east zone (Figure 2a). The largest discrepancy between urban and rural areas with regard to proportion of children seroprotected against tetanus was in the south west zone (Figure 2a). In comparison, the proportion of children <15 years with minimal or full diphtheria seroprotection was higher in rural compared to urban areas (*p* < 0.05) (Table 2). However, no significant difference was observed in the proportion of children who were seroprotected against diphtheria between rural and urban areas by geopolitical zone except for the south south and south west zones, where the proportion seroprotected was higher in rural areas (*p* < 0.05) (Figure 2b).

(**a**) Tetanus seroprotection among children aged <15 years by zone and location

(**b**) Diphtheria seroprotection among children aged <15 years by zone and location

**Figure 2.** Tetanus and diphtheria minimal seroprotection (≥0.01 IU/mL) among children aged <15 years by zone and location, Nigeria, 2018 (**a**,**b**).

The proportion of children <15 years who had minimal seroprotection against tetanus was higher in more wealthy quintiles, a pattern that was significant across all geopolitical zones (Figure 3a). In comparison, the proportion of children <15 years who had minimal seroprotection against diphtheria did not show a specific trend by wealth quintile. A significant difference in the proportion of children <15 years with minimal diphtheria seroprotection was observed by wealth quintile in the north central and south south geopolitical zones (*p* < 0.01) (Figure 3b).

**Figure 3.** Tetanus and diphtheria at least minimal seroprotection (≥0.01 IU/mL) among children aged <15 years by zone and wealth quintile, Nigeria, 2018 (**a**,**b**).

Among children aged <5 years, the proportion of children with minimal seroprotection for tetanus and diphtheria was 77.7% for children aged 4–11 months, 72.5% for 12–23 months, 66.2% for 24–35 months, 59.4% for 36–47 months, and 55.2% for 48–59 months, while the proportions of those children without seroprotection to both tetanus and diphtheria were 12.4%, 17.9%, 20.8%, 19.6%, and 19.0%, respectively. The proportion of children who had minimal seroprotection against diphtheria but not against tetanus increased with age from 5.8% among children aged 4–11 months, to 6.6% for 12–23 months, 9.4% for 24–35 months, 16.3% for 36–47 months, and 20.2% for 48–59 months. Overall, 13.2% of children aged 0–4 years, 20.7% of children aged 5–9 years, and 21.6% of children 10–14 years had minimal seroprotection to diphtheria but no seroprotection to tetanus.

By one-year age cohort, the proportion of children who were seroprotected against tetanus was lowest in children aged 2–10 years old, and waning immunity to tetanus was observed starting at the age of 3 years based on the drop in geometric mean antibody level (Figure 4a). The proportion of children with long-term tetanus seroprotection was highest in children aged younger than 1 year followed by 11–14 year-olds which corresponded to a similar trend in geometric mean antibody levels (Figure 4a). In comparison, geometric mean antibody levels and the proportions of children seroprotected against diphtheria were lowest among children aged 2–5 years but increased gradually among children aged 6–14 years (Figure 4b).

(**a**) Tetanus seroprotection levels and geometric mean antibody level by age

(**b**) Diphtheria seroprotection levels and geometric mean antibody level by age

**Figure 4.** Tetanus and diphtheria seroprotection levels and geometric mean antibody level by age, Nigeria, 2018 (**a**,**b**).
