Development of a cumulative metric of vaccination adherence behavior and its application among a cohort of 12-month-olds in western Kenya

Objectives The timely receipt of the recommended vaccination regimen, i.e. vaccination maintenance, is an underexplored, but important, indicator of public health. There is currently no standardized method for quantifying cumulative vaccination maintenance, however, and no simple way to explore predictors of adherence to vaccination schedules. We therefore sought to (1) develop a Vaccination Maintenance Score (VMS) and (2) apply this score to determine the predictors of vaccination behavior among infants in western Kenya (n = 245). Methods Women in western Kenya were enrolled during pregnancy and surveyed repeatedly through one year postpartum. Data were collected on a range of sociodemographic and health indicators and vaccinations. For each infant, we analyzed receipt of 11 vaccines recommended by the Kenyan Ministry of Health. We operationalized VMS as the total number of vaccines received on schedule. Vaccines that were not received or received off schedule were scored 0. VMS was modeled using multivariable tobit regression models. Results We found that 85.7% of infants were fully immunized, but only 42.4% had optimal VMS, i.e. scored 11. The median (IQR) VMS was 10 (3). In multivariable regression, each one-point increase in maternal quality of life score (range: 0–32) was associated with a 0.22-point increase in VMS; each additional child in the household was associated with a 0.34-point increase in VMS; and initiating breastfeeding at birth was associated a 2.01-point increase in VMS. Conclusions Coverage of the recommended vaccinations (85.7%) was nearly twice as high as cumulative timely receipt (42.4%). The VMS satisfies a need for a location-specific but easily adaptable metric of vaccination adherence behavior. It can be used to complement traditional methods of vaccination coverage and timeliness to better understand underlying behaviors that influence vaccination events, and thereby inform interventions to improve vaccination rates and decrease the burden of vaccine-preventable disease. Clinical Trial Registration NCT02974972 and NCT02979418.