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Hemagglutinin (HA) is essential for Influenza A virus infection, but its diversity of subtypes presents an obstacle to developing broad-spectrum HA inhibitors. In this study, we investigated the molecular mechanisms by which poly-galloyl glucose (pGG) analogs inhibit influenza hemagglutinin (HA) in vitro and in silico. We found that (1) star-shaped pGG analogs exhibit HA-inhibition activity by interacting with the conserved structural elements of the receptor binding domain (RBD); (2) HA inhibition depends on the number of galloyl substituents in a pGG analog; the best number is four; and when PGG binds with two HA trimers at their conserved receptor binding domains (loop 130, loop 220, and 190-α-helix), PGG acts as a molecular glue by aggregating viral particles so as to prevent viral entry into host cells (this was revealed via an in silico simulation on the binding of penta-galloyl-glucose (PGG) with HA). pGGs are also effective on a broad-spectrum influenza A subtypes (including H1, H3, H5, H7); this suggests that pGG analogs can be applied to most influenza A subtypes as a prophylactic against influenza viral infections. [ABSTRACT FROM AUTHOR]