2.Materials and methods
2.1.Viruses and clinical specimens
Stocks of US neurovirulence reference vaccine preparations with known virus titers were used as positive controls for the osRTPCR.They contained 109 50% tissue culture infectious doses permilliliter (TCID50/ml) of each serotype of the Sabin strains (GenBankaccession numbers: AY184219, AY184220, and AY184221).Thirty-one stool specimens were used to validate the osRT-PCR assay.These samples were collected in a previous study in which infants received different combinations of IPV and OPV (Asturiaset al., 2007).Written informed consent was obtained from a parent or guardian.