The results yield hope that vaccines, as part of larger flu-fighting efforts like early detection and quarantines, can be more effective in limiting the spread of disease.

"One of the main problems with vaccines is that people say that the virus can spread unnoticed, that it's still there although its effects can't be seen," van der Goot said. "We wanted to know if that's really happening or not, and we didn't find any transmission two weeks after vaccination."

"You have to do [the test] very carefully," van der Goot added. "If you miss only a small percentage of the birds, they can still become infected and [carry the] virus."

Hon Ip, director of the virology lab at the U.S. Geological Survey in Madison, Wisconsin, cautions that vaccination has demonstrated risks in the past.

"If there are variations in the quality of the vaccine, if it's not effective against a given strain, you might have partial protection," he said.

"A bird might not keel over and die, but it may survive and you might have the virus in that bird mutate to a different form. That's a situation that has occurred in Mexico since they began vaccinating against H5N2 in 1994."

Limited Tool

Ip also explained that avian influenza can be contracted from environmental exposure in contaminated areas, rather than from another animal.

"There was an outbreak at a poultry facility that they depopulated and tried to decontaminate," he said. "They found that they could recover the avian influenza virus from a liquid manure pit after 105 days—hardly ideal conditions for the virus to survive."

Another challenge is that the vaccine takes two weeks to provide full immunity. Bird flu spreads quickly, and even if it is detected early it could rapidly infect vaccinated birds during the two-week window before the drug becomes completely effective.

Despite the difficulties, vaccinations might save many flocks from the culls that have claimed tens of millions of birds in recent years. Van der Goot believes the vaccines could also serve as a field test for new drugs.

"We think these transmission experiments are good to evaluate the effectiveness of the vaccines on the market," she said. "[They are] a kind of test to see what's the best vaccine against a given strain."

Huaguang Lu, an avian virologist at Pennsylvania State University's College of Agricultural Sciences, noted that vaccination is a useful tool—but one that faces intrinsic limits.

"The important thing is that if you give a vaccination, you don't expect that you have a protection level forever," he explained. "For a few months, if you're near an outbreak and you need an emergency vaccination, it should work well.

"But once a flock [is infected] you have to eliminate the whole flock. You can't do vaccinations to save a flock, because the birds that are hosts will carry the virus, and you'll allow that virus more of a chance to mutate into a highly pathogenic or even [human-transmitted] strain."

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SOURCES AND RELATED WEB SITES National Geographic: Tracking the Next Killer Flu Proceedings of the National Academy of Sciences Central Institute for Animal Disease Control U.S. Geological Survey Pennsylvania State University: College of Agricultural Sciences