**2. Materials and Methods**

#### *2.1. Study Population and Sampling*

The target population included children younger than 15 years of age and residing in Nigeria. We used stored specimens collected during the 2018 Nigeria HIV/AIDS Indicator and Impact Survey (NAIIS) (https://www.naiis.ng/ (accessed on 15 January 2023). NAIIS was a national cross-sectional, household-based survey with a multi-stage cluster sampling design. In the first stage, clusters (enumeration areas) were selected using projected census data. Within each selected enumeration area, households were chosen through systematic sampling. Children less than 15 years of age were sampled in every 4th household (28,220 households sampled for inclusion of children aged 0–14 years). The eligibility criteria for children included having resided in the selected household or spent the night in that household before the survey; parents or guardians willing to provide written informed consent/permission in English, Hausa, Yoruba, or Igbo; and for children aged 10–14 years, the child is able and willing to provide written assent in English, Hausa, Yoruba, or Igbo. Further information on NAIIS methodology is available [22].

Assuming the most conservative estimate of 50% seroprevalence for tetanus and diphtheria with a 95% confidence interval, a design effect of 2, and a 5% non-response rate (e.g., not enough specimen), a sample size of 810 children per state (270 children each in age groups 0–4 years, 5–9 years and 10–14 years) would produce a precision of +/−9% for stateand age-level estimates. In total, 30,000 children would be needed from all 36 states and Abuja Federal Capital Territory (FCT) (10,000 specimens per age group). NAIIS collected specimens from 32,480 children ages 0–14 years, of which 32,337 (99%) assented for storage and future testing of specimens and were included in this study.
