 Influenza-specific IgG and sIgA ELISA: Influenza virus A specific IgG in serumplasma and sIgA in colostrum, breast milk and infant nasal swab will be analyzed by ELISA as described previously with minor modification Bruhl et al., 2000; Greenbaum et al., 2004. Briefly immunoplates will be coated with purified influenza antigen obtained from NIBSC at 2 ugmL for overnight at 40C. and then diluted samples will be added for another overnight incubation. HRP-conjugated anti-human IgG BD-Pharmingen, CA or mouse anti-sIgA, peroxidase-labeled Alpco Diagnostics, NH will be used as conjugates. To construct standard curve, a serial dilution of pooled samples assigned with an arbitrary value will be introduced in each ELISA plate. Total sIgA in colostrum, breast milk and infant nasal swab will also be determined by EIA kit Alpco Diagnostics to adjust influenza A specific sIgA response with respect to overall sIgA secretion. 