HA must form a trimer, where the subunits work together to bind to one sialic acid, before it undergoes a conformational change to create a protein channel through which the viral genetic material can enter the lung cell

HA must form a trimer, where each subunit can bind to a sialic acid, before it undergoes a conformational change to promote membrane fusion between the virus and lung cell

HA must form a trimer, where the subunits work together to bind to one sialic acid, before it undergoes a conformational change to promote membrane fusion between the virus and lung cell

HA must form a trimer, where each subunit can bind to a sialic acid, before it undergoes a conformational change to create a protein channel through which the viral genetic material can enter the lung cell