The 1918 influenza A virus caused the most devastating pandemic, killing approximately 50
million people worldwide. Immunization with 1918-like and classical swine H1N1 virus
vaccines results in cross-protective antibodies against the 2009 H1N1 pandemic influenza,
indicating antigenic similarities among these viruses. In this study, we demonstrate that
vaccination with the 2009 pandemic H1N1 vaccine elicits 1918 virus cross-protective
antibodies in mice and humans, and that vaccination or passive transfer of human-positive
sera reduced morbidity and conferred full protection from lethal challenge with the 1918
virus in mice. The spread of the 2009 H1N1 influenza virus in the population worldwide, in
addition to the large number of individuals already vaccinated, suggests that a large
proportion of the population now have cross-protective antibodies against the 1918 virus,
greatly alleviating concerns and fears regarding the accidental exposure/release of the 1918
virus from the laboratory and the use of the virus as a bioterrorist agent
