WASHINGTON - The H1N1 swine flu virus may have been new to humanity in many ways but in one key feature its closest relative was the 1918 pandemic virus, researchers reported on Wednesday.

Their findings could point to better ways to design vaccines and help explain why the swine flu pandemic largely spared the elderly.

"This study defines an unexpected similarity between two pandemic-causing strains of influenza," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases (NIAID), said in a statement.