Flublok contains recombinant HA proteins of the three strains of influenza virus specified by health
authorities for inclusion in the annual seasonal vaccine. These proteins function as antigens which induce a
humoral immune response, measured by hemagglutination inhibition (HI) antibody).
Antibodies against one influenza virus type or subtype confer limited or no protection against another.
Furthermore, antibodies to one antigenic variant of influenza virus might not protect against a new
antigenic variant of the same type or subtype. Frequent development of antigenic variants through
antigenic drift is the virologic basis for seasonal epidemics and the reason for the usual replacement of one
or more influenza virus strains in each year's influenza vaccine. Therefore, influenza vaccines are
standardized to contain the hemagglutinins of influenza virus strains (i.e., typically two type A and one type
B), representing the influenza viruses likely to be circulating in the U.S. in the upcoming winter.