A study was conducted in a part of the world where polio has not yet been eradicated, in order to estimate the efficacy of oral polio vaccine (OPV). From children with acute flaccid paralysis, two stool samples were obtained within 14 days of the onset of symptoms, and then tested for the presence of poliovirus. Parents of children who tested positive on at least one sample (cases) and negative on both samples (controls) were queried regarding immunization histories of the children. The results were as follows:

Although the examination of stool for the presence of poliovirus is believed to be 100% specific—no false positives—it probably is not 100% sensitive. Assume that among the 100 controls in the study, 10% truly were cases, irrespective of vaccination history. Estimate the vaccine effectiveness in the presence and the absence of this misclassification.