HA is the key viral antigen in determining host specificity and inducing neutralizing antibody since it plays a major role in binding to host cell receptors and fusing with host cell membranes [3] . For many challenging diseases caused by viruses, the recognition of certain neutralizing epitopes by the immune system can indeed provide broad and potent protection [4, 5] . The antigenic structure of HA and the corresponding antibody response are not fully understood, complicating rational design of vaccines aimed at modulating antibody responses for targeting key epitopes.