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. 2015 Dec 1;182(11):961-70.
doi: 10.1093/aje/kwv199. Epub 2015 Nov 14.

A New Method for Estimating the Coverage of Mass Vaccination Campaigns Against Poliomyelitis From Surveillance Data

K M O'Reilly, A Cori, E Durry, M Z Wadood, A Bosan, R B Aylward, N C Grassly

A New Method for Estimating the Coverage of Mass Vaccination Campaigns Against Poliomyelitis From Surveillance Data

K M O'Reilly et al. Am J Epidemiol. .

Abstract

Mass vaccination campaigns with the oral poliovirus vaccine targeting children aged <5 years are a critical component of the global poliomyelitis eradication effort. Monitoring the coverage of these campaigns is essential to allow corrective action, but current approaches are limited by their cross-sectional nature, nonrandom sampling, reporting biases, and accessibility issues. We describe a new Bayesian framework using data augmentation and Markov chain Monte Carlo methods to estimate variation in vaccination coverage from children's vaccination histories investigated during surveillance for acute flaccid paralysis. We tested the method using simulated data with at least 200 cases and were able to detect undervaccinated groups if they exceeded 10% of all children and temporal changes in coverage of ±10% with greater than 90% sensitivity. Application of the method to data from Pakistan for 2010-2011 identified undervaccinated groups within the Balochistan/Federally Administered Tribal Areas and Khyber Pakhtunkhwa regions, as well as temporal changes in coverage. The sizes of these groups are consistent with the multiple challenges faced by the program in these regions as a result of conflict and insecurity. Application of this new method to routinely collected data can be a useful tool for identifying poorly performing areas and assisting in eradication efforts.

Keywords: Bayesian analysis; acute flaccid paralysis; oral poliovirus vaccine; poliomyelitis; vaccination coverage.

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Figures

Figure 1.
Figure 1.
Schematic diagram of acute flaccid paralysis (AFP) data and the novel method the authors used for estimating oral poliovirus vaccine (OPV) immunization coverage in Pakistan. Lines representing the life histories of 7 children up to the onset of AFP are shown. The arrows at the top of the figure correspond to supplementary immunization activity (SIA), and the X's and diamonds for each child indicate whether the child was vaccinated during each SIA. Differences in the reported numbers of OPV doses received through SIA between children with AFP can be used to infer SIA coverage and is the basis for the new method described in the text (see Methods). In this example, 28.6% of children are reported to have received zero doses of OPV and no children are reported to have received more than 3 doses of OPV. In contrast to the new method, these summary statistics do not account for the age of each child, exposure to different numbers of SIAs, or errors in recall.
Figure 2.
Figure 2.
Age distribution of children with acute flaccid paralysis (AFP) not caused by poliomyelitis (nonpolio AFP) reported in Pakistan during 2010–2011 and their reported vaccination histories, by age. A) Age distribution; B) mean number of supplementary immunization activities (SIAs) experienced by children aged <2 years in different regions, by 6-month age group (black squares), together with the number of doses of oral poliovirus vaccine (OPV) they were reported to have received (boxes and whiskers). The boxes show the median value (black line) and interquartile range of the data, while the whiskers correspond to 1.5 times the interquartile range.
Figure 3.
Figure 3.
Summary statistics and estimates of oral poliovirus vaccine (OPV) immunization campaign coverage for specified regions of Pakistan, 2010–2011. The upper panels show the percentages of nonpolio acute flaccid paralysis (AFP) cases among children aged 6–23 months who were reported to have received more than 3 doses of OPV (A) and zero doses of OPV (B). Panel C presents crude estimates of vaccination coverage obtained by dividing the reported number of OPV doses by the number of immunization campaigns experienced by each child. Note that the upper limit can exceed 100%, reflecting error reporting. Panel D presents estimates of vaccination coverage obtained using the new method. In Balochistan/Federally Administered Tribal Areas (FATA) and Khyber Pakhtunkhwa (KP), the heterogeneous model gave the best fit, where 34% and 43% of children, respectively, were estimated to be in the undervaccinated group. Coverage in the undervaccinated group is shown by the black bars. The open diamonds indicate average coverage across the 2 groups in the heterogeneous model. The error bars in panels A–C indicate the 95% confidence intervals, and the error bars in panel D show the 95% credible intervals from the best-fit model.
Figure 4.
Figure 4.
Model estimates for oral poliovirus vaccine coverage and actual simulated coverage. A) Data simulated under the heterogeneous model with 40% in the undervaccinated group; B) data simulated under the homogeneous-temporal model. Estimates are shown by the triangles and squares, and the actual value in each group or time period is shown by the horizontal lines. Results from 50 simulations are shown.
Figure 5.
Figure 5.
Sizes and locations of persistently undervaccinated groups of children under age 5 years in Pakistan, 2010–2011. Data on cases of acute flaccid paralysis (AFP) were aggregated for groups of districts (A). The estimated proportions of children persistently undervaccinated are shown by the colored bars (B). Regional averages for Khyber Pakhtunkhwa (KP; North, Central, and South), Balochistan (North, Quetta, and Rest of Balochistan), and the Federally Administered Tribal Areas (FATA; Waziristan, Kyber, and Mohmand) are shown by the dashed horizontal lines. The solid error bars indicate the 95% credible interval for the percentage of children in the AFP sample who were in the undervaccinated group, and the dashed error bars indicate the 95% credible interval for the population estimate, accounting for the fact that children with AFP were a random sample from a larger population.
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