WASHINGTON (Reuters) - 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, said in a statement.

Two studies show an important structure called hemagglutinin is very similar in both the swine flu H1N1 and its distant cousin, the H1N1 virus that caused the 1918 pandemic. Hemagglutinin is used by viruses to infect cells and gives influenza viruses the "H" in their designations.

For one study, published in Science Translational Medicine, Chih-Jen Wei, Gary Nabel and colleagues at NIAID injected mice with a vaccine made using the 1918 influenza virus -- which killed an estimated 40 million to 100 million people.

When they infected the mice with H1N1 swine flu, the vaccinated mice survived, while many unprotected mice died.

The reverse also worked -- when they immunized mice using the 2009 H1N1 virus, and then infected them with the 1918 strain, the mice were protected.

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

The team also showed that as flu viruses circulate, they develop a kind of shield called a glycan that protects them from the body's immune system. That may allow them to become regular, seasonal visitors.

"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.

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