His new revelations have prompted the United States government to ask that the experiments be re-evaluated by a government advisory panel that recommended in December that certain details of the work be kept secret and not published, for fear that terrorists could use them to make bioweapons. Critics of the work had also warned that the virus might leak out of the lab accidentally and start a pandemic.
“We heard many times that this virus would spread like wildfire if it would come out of our facility,” said Ron Fouchier, the leader of the team that genetically altered the flu virus, at Erasmus Medical Center in Rotterdam, the Netherlands. “We do not think this is the case.”
Dr. Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases, said he also thought that the danger might have been overstated. “There is a gross, pervasive misunderstanding out there,” he said, adding that he had recommended that the data be examined again by the National Science Advisory Board for Biosecurity, overseen by the National Institutes of Health. He said the board would probably reconvene in March.
The experiments involve a type of bird flu virus known as H5N1. It does not often infect people, but appears unusually deadly when it does. Of about 600 known cases since 1997, more than half have been fatal. The exact death rate is not known because some deaths and mild cases may go uncounted. But most researchers think the virus is more deadly than other flu viruses, even the notorious 1918 flu, which killed as many as 50 million people worldwide. But the 1918 flu was highly contagious; so far, bird flu has rarely spread from person to person. People who fall ill have nearly always caught it from poultry.
Dr. Fouchier’s experiments involved ferrets, which are considered a good model for flu research because they react to the virus in much the same way that people do. Researchers can infect ferrets with H5N1 by squirting the virus into their noses or lungs, but then the animals normally do not infect one another. Dr. Fouchier’s team altered the virus genetically so that it could become airborne, and spread from one ferret to another by coughing and sneezing.
Until recently, Dr. Fouchier had not revealed much about what happened to the infected ferrets, in part because he had agreed to keep the details secret until decisions could be made about how much of it to make public. But on Wednesday, at a meeting in Washington of the American Society for Microbiology, he said that when healthy ferrets were exposed to the coughs and sneezes of infected ones, not all became infected. Those that did become infected did not get very sick or die. In addition, he said, if the ferrets were previously exposed to a run-of-the-mill seasonal flu, they were immune to the bird flu.
Most adult humans have had some type of flu, and therefore, Dr. Fouchier said, they may also have some natural protection against bird flu.
But Michael Osterholm, a member of the biosecurity board, warned that ferrets were not a perfect model for what would happen in humans, and that it was impossible to tell how virulent or contagious the new virus would be in people. He said that the board would review any new information with great care and caution.
A version of this article appeared in print on March 1, 2012, on page A6 of the New York edition with the headline: Genetically Altered Bird Flu Virus Not as Dangerous as Believed, Its Maker Asserts.
**Published in "THE NEW YORK TIMES"