During membrane fusion, the outer leaflets of the two membranes merge first, whereas the distal membrane leaflets remain separate until the opening of a fusion pore. This intermediate stage, called hemifusion, is a critical event shared by exocytosis, protein trafficking, and viral entry. Copyright ©2005 by Elsevier Inc.
Chernomordik, L. V., & Kozlov, M. M. (2005, November 4). Membrane hemifusion: Crossing a chasm in two leaps. Cell. https://doi.org/10.1016/j.cell.2005.10.015