On Wednesday, researchers reported that the H1N1 swine flu virus might have been new to humanity in lot of ways, but in one key characteristic its closest relation was the 1918 epidemic virus.

Their results could indicate better ways to make vaccines and help elucidate why the swine flu virus mostly spared the elderly.

Dr. Anthony Fauci, Director of the National Institute of Allergy and Infectious Diseases, said, "This study defines an unexpected similarity between two pandemic-causing strains of influenza".

Two studies exhibit a significant structure called hemagglutinin, which is very similar in both the swine flu H1N1 and its distant kin, the H1N1 virus that caused the 1918 epidemic. Hemagglutinin is employed by viruses to contaminate cells and gives influenza viruses the "H" in their titles.

In one of the research, published in Science Translational Medicine, Chih-Jen Wei, Gary Nabel and coworkers at NIAID injected mice with a vaccine prepared using the 1918 influenza virus, which killed an approximately 40 million to 100 million people.

When the mice were infected with H1N1 swine flu, the immunized mice survived, while several unprotected mice succumbed to death.

Similarly, when they vaccinated mice with the 2009 H1N1 virus, and then infected them with the 1918 strain, the mice were protected.

Nabel said, "This is a surprising result. We wouldn't have expected that cross-reactive antibodies would be generated against viruses separated by so many years".