Do inequalities exist in the disadvantaged populations? Levels and trends of full and on-time vaccination coverage in two Nairobi urban informal settlements

There has been an improvement in childhood vaccination coverage over the last two decades worldwide. However, inequities exist among different populations. Vaccination programs should focus not only on increasing coverage but as also in timeliness to ensure maximum protection. This study examined the levels, inequities, and trends of full and on-time vaccination coverage in two urban informal settlements in Nairobi. The study used longitudinal data from the Nairobi Urban Health and Demographic Surveillance System from 2003 to 2017 to estimate full and on-time vaccination coverage and assess inequalities by background characteristics. The frailty shared Cox model was used to assess time to full- and on-time- immunization coverage. Out of 32,018 children aged 12 to 59 months, less than half (46.7%) produced a vaccination card during the interview. Full and timely immunization coverage was higher in Viwandani site, among Kikuyu and Kamba ethnic groups, and children from the richest quintile. Timely vaccination was below 50% throughout the survey periods. After accounting for the intragroup correlations, for a given level of frailty, the hazard for being fully immunized was 10% more likely among the wealthiest compared to the poorest children. The hazard for being fully immunized was 16%, 16% to 19% less likely for Luhya, Luo, and others as compared to the Kikuyu ethnicity respectively. In conclusion, the study has shown that coverage has been increasing over the years but inequalities exist in immunization coverage among the most disadvantaged populations. More focused intervention approaches that target the disadvantaged groups are needed.