Spatial variability and uncertainty for under-vaccination and zero-dose in fragile settings: A model-based study of immunization coverage in Chad

Universal access to childhood vaccination is important to child health and sustainable development. Here we identify, at a fine spatial scale, under-immunized children and zero-dose children. Using Chad, as an example, the most recent nationally representative household survey that included recommended vaccine antigens was assembled. Age-disaggregated population (12-23 months) and vaccination coverage were modelled at a fine spatial resolution scale (1km x 1 km) using a Bayesian geostatistical framework adjusting for a set of parsimonious covariates. There was a variation at fine spatial scale in the population 12-23 months a national mean of 18.6% (CrI 15.8% – 22.6%) with the highest proportion in the South-East district of Laremanaye 20.0% (14.8 – 25.0). Modelled coverage at birth was 49.0% (31.2% - 75.3%) for BCG, 44.8% (27.1 – 74.3) for DPT1, 24.7% (12.5 – 46.3) for DPT3 and 47.0% (30.6 – 71.0) for measles (MCV1). Combining coverage estimates with the modelled population at a fine spatial scale yielded 312,723 (Lower estimate 156,055 - 409,266) zero-dose children based on DPT1. Improving routine immunization will require investment in the health system as part of improving primary health care. Uncertainty highlights areas requiring further investigation and quality data to better understand coverage.