Abstract:
Background: The benefits of childhood vaccines are critically dependent on vaccination
coverage. We used a vaccine registry (as gold standard) in Kenya to quantify errors in
routine coverage methods (surveys and administrative reports), to estimate the magnitude
of survivor bias, contrast coverage with timeliness and use both measures to estimate
population immunity.
Methods: Vaccination records of children in the Kilifi Health and Demographic
Surveillance System (KHDSS), Kenya were combined with births, deaths, migration and
residence data from 2010 to 17. Using inverse survival curves, we estimated up-to-date
and age-appropriate vaccination coverage, calculated mean vaccination coverage in
infancy as the area under the inverse survival curves, and estimated the proportion of
fully immunised children (FIC). Results were compared with published coverage
estimates. Risk factors for vaccination were assessed using Cox regression models.
Results: We analysed data for 49,090 infants and 48,025 children aged 12-23 months in 6
birth cohorts and 6 cross-sectional surveys respectively, and found 2nd year of life
surveys overestimated coverage by 2% compared to birth cohorts. Compared to mean
coverage in infants, static coverage at 12 months was exaggerated by 7-8% for third
doses of oral polio, pentavalent (Penta3) and pneumococcal conjugate vaccines, and by
24% for the measles vaccine. Surveys and administrative coverage also underestimated
the proportion of the fully immunised child by 10-14%. For BCG, Penta3 and measles,
timeliness was 23-44% higher in children born in a health facility but 20-37% lower in
those who first attended during vaccine stock outs.
Conclusions: Standard coverage surveys in 12-23 month old children overestimate
protection by ignoring timeliness, and survivor and recall biases. Where delayed
vaccination is common, up-to-date coverage will give biased estimates of population
immunity. Surveys and administrative methods also underestimate FIC prevalence.
Better measurement of coverage and more sophisticated analyses are required to control
vaccine preventable diseases.