The findings, published in the journal Clinical Infectious Diseases, underlines the importance of using antibodies from recovered patients to treat critically ill people who fail to respond to standard drugs, said lead author Kwok-yung Yuen, a virologist at the University of Hong Kong.

"Most (severely ill patients) come to hospital very late, on day 5 or 7 (after onset of symptoms). Our experience has been that antiviral drugs don't work very well," said Yuen.

"That's why convalescent plasma (antibodies) would have a place in saving patients who are very severely ill and not responding to Tamiflu," he told Reuters.

Swiss drug maker Roche's Tamiflu, under license from Gilead Sciences, is the drug of choice to fight the pandemic H1N1 flu virus.

Seasonal flu kills between 250,000 to 500,000 people each year globally and the H1N1 swine flu may have been slightly more deadly, but actual statistics will take years to gather. It affects younger adults and children more severely compared to seasonal flu, which kills more elderly people.