The HI test quantitates serum antibody to influenza virus which can prevent agglutination of turkey RBCs (Fitzgerald Industries International Inc., MA). Heat-inactivated serum samples were treated with receptor-destroying enzyme (Sigma-Aldrich) for removing nonspecific inhibitors (followed by RBC adsorption) and were diluted 2-fold serially from initial dilution of 1:10. HA antigen (8 HA units in 25 mL) were added onto each well and incubated for 1 h at RT. Following antigen-antibody reaction, 50 mL of 0.5% turkey RBC were added to each well and incubated for 1 h at RT. HI negative wells were scored based upon a diffuse sheet of agglutinated RBCs covering the bottom. HI positive wells were scored if they showed a well circumscribed button of nonagglutinated RBCs.