IAV binding to secreted mucus is Sia-dependent. Human trachea tissue sections were treated with Arthrobacter ureafaciens sialidase (AUS), which cleaves Sias, or with mild sodium periodate (NaIO4), which truncates the Sia side chain. Both treatments reduce IAV binding to the secreted mucus on human trachea tissues compared to untreated control tissues, confirming that IAV binding to the secreted mucus is Sia-dependent. Dashed lines specify location of virus binding to secreted mucus. Scale bar indicates 50 μm.