Taken together, these data underline the importance of PN-SIA49 for the development of anti-influenza strategies based on passive immunization. Further studies will be necessary to define the most effective prophylactic and therapeutic administration protocols. Finally, these data and a more detailed definition of the epitope recognized by PN-SIA49 could also be useful to develop new vaccination strategies able to elicit a humoral immune response directed against key regions of the influenza HA protein.