PF4 and infection: The S protein can induce strain-specific immunity in biologically-produced vaccines; it can also induce antibody-dependent enhancement (ADE) (11) of infection (Figure 1C, dark blue region), vaccine-associated respiratory disease (VAERD) (12) and some other very severe adverse secondary reactions, like the recently-described predisposition to acute thrombosis and thrombocytopenia (13) associated with cross reactivity with platelet factor 4 (PF4) (14).