Haemagglutinin-strain B

Haemagglutinin-strain B is a purified antigenic protein derived from influenza B virus strains, commonly used as a component in seasonal influenza vaccines. It is not itself a standalone drug but rather one of the viral antigens included in quadrivalent and trivalent inactivated influenza vaccines. The primary mechanism of action involves inducing humoral immune responses—specifically, the production of neutralizing antibodies against the haemagglutinin protein on the surface of influenza B viruses. These antibodies can prevent infection by neutralizing circulating viruses with matching antigens[5][8]. Haemagglutinin is responsible for viral entry into host cells by binding to sialic acid receptors and mediating membrane fusion[6]. The inclusion of both Victoria and Yamagata lineages (strains) ensures broader protection against circulating influenza B variants[8]. Immunity typically develops within 2–3 weeks post-vaccination, but annual revaccination is recommended due to waning immunity and antigenic drift among circulating strains[5][8].