A team at Retroscreen Virology, an institute associated with the University of London, said that the extract was at least 99 per cent effective against the H5N1 virus and significantly neutralized the infectivity of the virus in cell cultures.
Dr Madeleine Mumcuoglu, the Israel-based developer of Sambucol, said the good results confirmed her earlier findings on the product, which is currently marketed around the world as a natural supplement for fighting common flu.
"It is active against lots of different types of flu viruses and I did my PhD on its effect on a non-virulent strain of bird flu so the results did not surprise me," she told NutraIngredients.com.
"But I'm very happy with these results, especially as the work was done by Dr John Oxford, a reputable specialist in flu," she added.
Dr Mumcuoglu stressed that the results could in no way demonstrate a benefit for humans infected with the bird flu strain that has killed more than 70 people, mostly in South East Asia, since the outbreak began in 2003. Nor will she be able to test it in a human trial.
But she is looking into starting an animal trial using ferrets, which have similar flu symptoms to humans.
Dr Mumcuoglu added that bird flu is still a lower threat than common flu, thought to kill about 25,000 people in the UK alone each year through complications.
While the new bird flu results might boost sales of Sambucol, it is the evidence from two human clinical trials on human flu that has created a significant market for the product. The double blind, placebo-controlled studies - the first done in Israel, and the second in Norway - both found that people who took the elderberry supplement got rid of flu in half the time of the placebo group, or after two to three days compared to about six.
Dr Mumcuoglu says Sambucol blunts the haemaglutinin spikes on the outside of viruses and stops them from entering cells where they reproduce, cause the cell to explode and allow the virus to continue invading the body.
An in vitro study has also shown Sambucol to be effective in increasing the production of four inflammatory cytokines, suggesting that the supplement may have an immuno-stimulatory effect and therefore be worth taking all year round to prevent flu and other disease.
Jerusalem-based Razei Bar Industries now produces more than 1.5 million bottles of Sambucol syrup, with average growth of 10 per cent each year.
"I didn't invent anything," claims Dr Mumcuoglu. "Elderberry has been known from the beginning of time. What people didn't know is what the active ingredient is, how to keep it intact and how to formulate to keep it active."
This active substance has not been patented to protect the firm's market. While several copycat products are available in healthstores, Razei Bar says it has tested several of these and found none of the active ingredient.
The new laboratory results were presented at a press conference held in the Royal Society of Medicine, London last week. Another trial is now underway at Hadassah Medical Organization Ein Kerem in Israel.