Using geospatial models to map zero-dose children: factors associated with zero-dose vaccination status before and after a mass measles and rubella vaccination campaign

Abstract

AbstractIntroductionDespite gains in global coverage of childhood vaccines, many children remain undervaccinated. Vaccination campaigns also known as Supplemental Immunization Activity (SIA) are commonly conducted to reach those who are undervaccinated. However, reaching these children even during an SIA is challenging. We evaluated the effectiveness of an SIA in reaching zero dose children.MethodsWe conducted a prospective study in 10 health center catchment areas in Southern province, Zambia in November 2020. About 2 months before the measles and rubella SIA we developed aerial satellite maps which were then used to enumerate and survey households. Zero dose children were identified during this exercise. After the SIA, households with zero dose children identified before the SIA were targeted for mop up vaccination and to assess if they were vaccinated during the SIA. A Bayesian geospatial model was used to identify factors associated with zero-dose status before the campaign and produce fine-scale prevalence maps. Models were used to identify factors associated with measles zero-dose children reached in the campaign and identify optimal locations for additional vaccination sites.ResultsBefore the vaccination campaign, 4% of children under 9 months were DTP zero-dose and 17% of children 9-60 months were measles zero-dose. Of the 461 measles zero-dose children identified before the vaccination campaign, 338 (73.3%) were vaccinated during the campaign, 118 (25.6%) were reached by a targeted mop-up activity. The presence of other children in the household, younger age, greater travel time to health facilities, and living between health facility catchment areas were associated with zero-dose status. Mapping zero-dose prevalence revealed substantial heterogeneity, both within and between catchment areas. Several potential locations were identified for additional vaccination sites.ConclusionFine-scale variation in zero-dose prevalence and the impact of accessibility to healthcare facilities on vaccination coverage were identified. Geospatial modeling can aid targeted vaccination activities.SummaryWhat is already known?In many low- and lower-middle income countries, improvements in routine childhood vaccination coverage have stalled.An estimated 17 million children globally have not received any routine vaccinations (zero-dose children).Zero-dose children, and those who have not received any doses of specific vaccines such as measles, remain vulnerable to preventable diseases and can sustain transmission in otherwise highly vaccinated populationsA lack of understanding of the number and spatial distribution of zero-dose children make targeting vaccination activities to reach this group challenging.What are the new findings?Prior to a mass measles and rubella vaccination campaign, 17% of children younger than 9 months of age in the study area had not received the DTP vaccine and 4% of children 9 months or older had not received a measles-containing vaccine.Over a quarter of the children identified as not having received a measles-containing vaccine before the measles and rubella mass vaccination campaign were not vaccinated during the campaign.Geospatial models revealed substantial fine-scale variation in zero-dose status and optimal locations for additional vaccination sites.What do the new findings imply?There is potential for using similar household-level geospatial survey and modeling strategies to improve targeting of vaccination activities to reach zero-dose children.