than any other single pathogen, except malaria, and poses a substantial global health
burden. Viral entry is mediated by a type I fusion glycoprotein (F) that transitions from a
metastable prefusion (pre-F) to a stable postfusion (post-F) trimer. A highly neutralization-
sensitive epitope, antigenic site Ø, is found only on pre-F. We determined what fraction of
neutralizing (NT) activity in human sera is dependent on antibodies specific for antigenic site …