With the addition of supplemental ES, if ES is beneficial we would expect to observe 1) fewer gaps in the viral phylogeny as measured by detection of fewer ‘orphan’ isolates (defined below) and 2) prior to the onset of each polio case, genetically-similar isolates would be detected by ES sooner on average than genetically-related isolates detected by AFP surveillance. The gene discussed is AFP; the disease is poliomyelitis.