A theoretical framework for evaluating the sensitivity of surveillance for detecting wild poliovirus: II. Factors affecting detection sensitivity in a population with circulating wild poliovirus.

A basic framework for describing sensitivity of surveillance to detect poliovirus is extended from individuals to general populations (population sensitivity). Using the mathematical formulations for population sensitivity, the theoretical behavior of surveillance sensitivity under various conditions is analyzed. As a region nears the elimination of poliovirus, population sensitivity falls to a value lower than the case-to-infection ratio, regardless of the system proficiency. Also, a second stool specimen makes a substantial contribution to population sensitivity only in regions with relatively low specimen sensitivity and then only over a narrow range of population infection incidence. Estimates of the mean numbers of infected and uninfected acute flaccid paralysis cases investigated in a season are derived. These may serve as additional indicators of system operation but require the collection of 2 specimens per case.