**1. Introduction**

Historically, vaccination coverage surveys have been used to assess the performance of immunization programs and identify areas at risk for vaccine-preventable diseases (VPDs). However, data have shown that VPD coverage surveys have limitations due to poor documentation of immunization history and parental recall bias [1]. In settings with weak surveillance or unreliable vaccination coverage, serological surveillance can potentially play an important role for appropriately directing interventions to improve population immunity [2,3]. Serological surveys are increasingly being used to guide immunization policy and strategy from support of vaccine introduction, evidence generation for optimizing timing of booster doses, to the verification of disease elimination [2–14]. In the case of tetanus, serological surveys can also assess routine vaccination coverage

**Citation:** Tohme, R.A.; Scobie, H.M.; Okunromade, O.; Olaleye, T.; Shuaib, F.; Jegede, T.; Yahaya, R.; Nnaemeka, N.; Lawal, B.; Egwuenu, A.; et al. Tetanus and Diphtheria Seroprotection among Children Younger Than 15 Years in Nigeria, 2018: Who Are the Unprotected Children? *Vaccines* **2023**, *11*, 663. https://doi.org/10.3390/ vaccines11030663

Academic Editor: Davide Gori

Received: 16 February 2023 Revised: 9 March 2023 Accepted: 13 March 2023 Published: 15 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

as tetanus infection does not lead to development of protective antibodies [14]. Tetanus serosurveys can identify areas or subgroups yet to be reached with routine immunization and also assess duration of vaccine-induced immunity to inform introduction of tetanus (and diphtheria) booster doses [14]. Depending on the disease, serum antibody levels can be maintained for years following a person's vaccination or exposure to a pathogen, like the one that causes diphtheria; therefore, specimens collected during cross-sectional surveys contain an immense amount of information about current and past pathogen exposure and levels of immunity [4].

High tetanus burden in Nigeria has been documented [15], with the tetanus case fatality ratio (CFR) estimated at 43% [16]. In addition, high diphtheria burden and recurring diphtheria outbreaks with high CFR have been documented in Nigeria, including a recent outbreak in 2023 [17–19]. In 2011, over 60% of diphtheria cases occurred among children younger than 10 years of age and over 95% were unvaccinated [17]. Nigeria's immunization schedule includes three doses of diphtheria-tetanus-pertussis (DTP) containing vaccine at 6, 10, and 14 weeks of age. However, diphtheria and tetanus immunity wane over time and by school age (5–6 years old), many children are susceptible to infection. For this reason, in addition to the three primary DTP (DTP3) doses given before the age of one year, WHO recommends three booster doses of tetanus- and diphtheria-containing vaccines be provided to children and adolescents at the ages of 12–23 months, 4–7 years and 9–15 years to provide protection across the life-course [20,21].

Data on tetanus and diphtheria immunity among children in Nigeria are needed to evaluate susceptibility to VPDs and for use in program improvement to prevent tetanus cases and diphtheria epidemics. We conducted a national serological assessment to estimate immunity to tetanus and diphtheria among Nigerian children born during 2004–2018 (aged <15 years old at the time of the survey in 2018) and by subpopulation (age, sex, zone, state, urban/rural, and wealth quintile) to identify immunity gaps and specific populations that might need targeted interventions to improve vaccination coverage.
