The flu virus of 1918 and last year's H1N1 strain has been found to be quite similar, and this may help in the creation of new vaccines.
The research was published in the online journal Science Translational Medicine said.
"This study defines an unexpected similarity between two pandemic-causing strains of influenza," said Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases (NIAID).
"It gives us a new understanding of how pandemic viruses evolve into seasonal strains, and, importantly, provides direction for developing vaccines to slow or prevent that transformation," Fauci said.
Researchers at the NIAID and the US Centers for Disease Control and Prevention (CDC) injected mice with a vaccine made from inactivated 1918 influenza virus, then exposed them to high levels of the 2009 H1N1 virus.
All of the vaccinated mice survived.
The scientists then repeated the experiment, this time vaccinating the mice with inactivated 2009 H1N1 virus and exposing them to high concentrations of the 1918 virus. Again, the mice survived.
The "surprising" finding is that either pandemic virus could be used to create a vaccine that caused the mice to produce antibodies capable of neutralizing the other.
"This is a surprising result," said NIAID scientist Gary Nabel, who led the research.
"We wouldn't have expected that cross-reactive antibodies would be generated against viruses separated by so many years," he said.