Seroconversion of chickens in the transmission experiments. In every independent experiment, sera of ten chickens in every group were collected at 7 day intervals and seroconversion was confirmed by hemagglutination inhibition (HI) assay. Each color bar represents the antibody titers of every chicken. Repeat seroconversion experiments of r01/NASS and r01/NA381 were performed. The results also indicate that no seroconversion was observed in aerosol contact chickens. 1–10 represents ten inoculated chickens, 11–20 represents ten direct-contact chickens and 21–30 represents ten aerosol-contact chickens. The blue, red and green colors express antibody titers detected on 7, 14 and 21 dpi respectively.