 Hemagglutination inhibition test HAI: HAI antibody titers will be measured by standard microtiter assay Admon et al., 1997. 12 HA units of the selected Influenza A vaccine strain will be added to diluted pre-treated sera with receptor destroying enzyme RDE, Sigma MO and heat inactivation. Following incubation, washed chicken erythrocytes will be added until sedimentation ensues in the control wells. HAI titer is the inverse of highest dilution of serum exhibiting complete HAI. 