There are 371 females and 545 males

There are 371 females and 545 males. All serum samples were treated with a receptor-destroying enzyme (RDE) and absorbed with erythrocytes in order to remove nonspecific inhibitors before detection. All samples were tested with a hemagglutination inhibition (HI) assay according to recommended procedures as reported as described previously. Briefly, two-fold serial dilutions of serum samples were added into V-shaped microtitre plate, followed with addition of 4 HA units of virus into each well. The mixture was incubated at room temperature for 35 min. Then, 1% (v/v) chicken red blood cells (RBCs) were added into each well. The plates were left at room temperature for 40 min.