ZURICH, Sept 19 (Reuters) - A vaccine produced by Novartis <NOVN.VX> induces a protective antibody level against diverse strains of bird flu in individuals already vaccinated six years earlier, a study presented on Friday showed.

Novartis said the study conducted by Britain's Leicester University showed that its MF59-adjuvanted booster vaccine triggers an immune response to the H5N1 strain of bird flu, which experts say could trigger a human flu pandemic.

"The immune response was broadly cross reactive and covered all H5N1 clades known to date," Novartis said, adding the data were presented at the European Influenza Conference in Portugal.

Leicester University study investigator Iain Stephenson said: "These results potentially provide a rationale to prevent pandemic influenza by proactively immunizing the public with stockpiled pre-pandemic vaccines containing MF59."

In May, European authorities approved the first pre-pandemic bird flu vaccine, Prepandrix, from GlaxoSmithKline Plc <GSK.L>, which hopes the move will spur fresh stockpile orders from governments around the world.

Industry analysts say bird flu vaccines may represent a $1 billion-plus sales opportunity for such companies.

While H5N1 remains mainly a virus affecting birds, scientists say it is the most likely source of the next global flu pandemic in humans, since it may soon mutate into a form that is easily transmitted from person to person.

So far, most human cases can be traced to direct or indirect contact with infected birds. The H5N1 virus has killed around 240 people since 2003. (Reporting by Emma Thomasson; Editing by David Cowell)

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