Quantifying the Impact of Expanded Age Group Campaigns for Polio Eradication

<div><p>A priority of the Global Polio Eradication Initiative (GPEI) 2013–2018 strategic plan is to evaluate the potential impact on polio eradication resulting from expanding one or more Supplementary Immunization Activities (SIAs) to children beyond age five-years in polio endemic countries. It has been hypothesized that such expanded age group (EAG) campaigns could accelerate polio eradication by eliminating immunity gaps in older children that may have resulted from past periods of low vaccination coverage. Using an individual-based mathematical model, we quantified the impact of EAG campaigns in terms of probability of elimination, reduction in polio transmission and age stratified immunity levels. The model was specifically calibrated to seroprevalence data from a polio-endemic region: Zaria, Nigeria. We compared the impact of EAG campaigns, which depend only on age, to more targeted interventions which focus on reaching missed populations. We found that EAG campaigns would not significantly improve prospects for polio eradication; the probability of elimination increased by 8% (from 24% at baseline to 32%) when expanding three annual SIAs to 5–14 year old children and by 18% when expanding all six annual SIAs. In contrast, expanding only two of the annual SIAs to target hard-to-reach populations at modest vaccination coverage—representing less than one tenth of additional vaccinations required for the six SIA EAG scenario—increased the probability of elimination by 55%. Implementation of EAG campaigns in polio endemic regions would not improve prospects for eradication. In endemic areas, vaccination campaigns which do not target missed populations will not benefit polio eradication efforts.</p></div

The distribution of mucosal immunity in 5–14 year-olds in EAG campaigns compared with standard campaigns targeting 0–4 year-olds.

<p>Average mucosal immunity waning time was varied between 6 months and 10 years (half-life 0.34–7 years), and SIA coverage between 10%–60% per round. We reported the results in fractions of mucosal antibody titer >8, assuming a fixed fraction of acquired viral dose, . We note that other values of yield similar results: (<b>A</b>) no wild poliovirus circulation, baseline; (<b>B</b>) no wild poliovirus circulation, expanded age group campaigns; (<b>C</b>) with wild poliovirus circulation (), baseline scenario; and (<b>D</b>) with wild poliovirus circulation (), expanded age group campaigns.</p

Distribution of cases by age (in years) during previous polio outbreaks for endemic and previously polio-free countries.

<p>The number of confirmed type 1 cases, mean age of infection, standard deviation of infection age, proportion of cases under five years old, and duration of case data used are in the table to right. High endemic countries are those that have sustained continuous transmission: India (IND), Afghanistan (AFG), Pakistan (PAK), and Nigeria (NGA). Low endemic areas are those that are exposed periodically to virus due to regular importations: Chad (TCD) and Niger (NER). Importation countries are those that have not reported WPV transmission since at least 2000: Democratic Republic of Congo (COD), Namibia (NAM), Tajikistan (TJK), and Republic of Congo (COG). Information compiled from multiple AFP databases maintained by WHO HQ, regional and country offices.</p

Model calibration and the effect of both expanding age groups and targeting in SIA campaigns.

<p>(A) Likelihood of polio infectiousness parameters: relative probability of reproducing the observed seroprevalence data from a sample of the model results conducted in the same manner (number of samples by age) as in the original Zaria serosurvey. Each tick mark represents a two-fold change in likelihood. (<b>B</b>) The effect of expanded age group SIA campaigns on elimination: distributions of mean WPV1 prevalence for baseline (calibration), EAG campaigns, and a campaign targeting hard-to-reach groups.</p

Effect of expanding age groups in SIA campaigns on mucosal immunity.

<p>(<b>A</b>) Mucosal antibody titer distribution before and after expanded age group campaigns in the overall population (5–9 years) and (<b>B</b>) the unvaccinated group (5–9 years) only. log₂(mucosal antibody titer)<3 represents high susceptibility to infection.</p

Temporal dynamics of WPV1 prevalence for the most likely set of calibration parameters under baseline conditions, EAG campaigns, and outreach campaigns.

<p>Temporal dynamics of WPV1 prevalence for the most likely set of calibration parameters under baseline conditions, EAG campaigns, and outreach campaigns.</p

Behavior of the polio model.

<p>(<b>A</b>) Distribution of mucosal immunity by number of OPV doses given to a previously naïve individual. The depth axis in figure represents the probability distribution of antibody titer in the stochastic model and the red line represents the population mean of the antibody titer resulting from the distribution. (<b>B</b>) Shedding as a function of initial mucosal immunity and time since infection. (<b>C</b>) Probability of infection with different immunity levels and challenge doses.</p