**5. Conclusions**

This study provided the first estimates of tetanus and diphtheria seroprotection in Nigeria. The findings highlighted the need to target specific geographic areas and to consider gender and socio-economic equity in improving access and demand for vaccinations, as well as the need to introduce tetanus and diphtheria containing booster doses at 12–23 months, 4–7 years, and 9–15 years of age to address waning immunity and prevent tetanus cases and diphtheria outbreaks. The survey results also highlighted the benefit of supplementing vaccination coverage data with seroprevalence data and other data sources to provide a better assessment of the vaccination program and to help identify immunity gaps.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/vaccines11030663/s1, Table S1: Long term tetanus and diphtheria seroprotection among children aged <15 years, Nigeria 2018.

**Author Contributions:** All authors meet the four criteria for authorship in the ICMJE Recommendations. Conceptualization, R.A.T. and H.M.S.; Formal analysis, H.M.S., T.O., T.J., R.Y., N.N., B.L., A.E., N.P., G.C., Q.A. and M.C.; Funding acquisition, R.A.T., H.M.S. and O.B.; Methodology, R.A.T., H.M.S., G.C. and Q.A.; Supervision, R.A.T., F.S., O.B., I.A. and C.I.; Validation, N.P., G.C. and M.C.; Writing—original draft, R.A.T.; Writing—review and editing, H.M.S., O.O., T.O., F.S., T.J., R.Y., N.N., B.L., A.E., N.P., G.C., Q.A., M.C., O.B., B.B.O., I.A. and C.I. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the U.S. Centers for Disease Control and Prevention.

**Institutional Review Board Statement:** The study was conducted in accordance with the Declaration of Helsinki, and approved by the National Health Research Ethics Committee of Nigeria (NHREC) (protocol code NHREC/01/01/2007, date of approval: 27 August 2019) and the Human Subjects Review Board at the US Centers for Disease Control and Prevention where it was determined to be non-research as the study involved testing previously collected de-identified specimens.

**Informed Consent Statement:** Written informed consent to use specimens to test for diseases other than HIV was obtained from all subjects involved in the study.

**Data Availability Statement:** Data are unavailable due to privacy consideration as datasets include global positioning system coordinates which might enable identification of location of study subjects. Only restricted individuals had access to the datasets for analyses purposes only.

**Acknowledgments:** Erasogie Evbuomwan, Nigeria Center for Disease Control; Oluwaseun Akinmulero, Mudiaga Esiekpe Kayode, Institute of Human Virology, Abuja, Nigeria; Gavin Grant, Stacie Greby, Ryan Wiegand, Barbara Marston, Diana Martin, Mahesh Swaminathan, Ibrahim Dalhatu, Matthias Alagi, McPaul Okoye, Centers for Disease Control and Prevention; and the NAIIS project team and respondents.

**Conflicts of Interest:** The authors declare no conflict of interest.
