The lack of established correlates of immunity in animals and humans poses challenges to developing consistent immunologic endpoints for clinical trials and appropriate criteria for vaccine effi cacy. Serum antibody titers, mainly those determined by hemagglutination inhibition (HI) or virus neutralization (VN) assays, or both, are considered surrogate measures of protection. However, the HI test is insensitive for the detection of antibody to avian HA; there also are no recognized clinical correlates of immune protection for neutralization antibody (33, 34) . Recently, HI or VN assay failed to detect antibodies in ferrets protected by vaccination with whole-virus vaccines containing internal protein from Dk/Sing virus against a heterotypic virus (34). Whether the cross-protection reported is mediated by Tcell response is not known.