While the airborne spread wasn't highly efficient, the work suggests this virus is more closely adapted to person-to-person spread than other bird flu viruses, the authors said.
"Under appropriate conditions human-to-human transmission of the H7N9 virus may be possible," the scientists — from China, Hong Kong, the United States and Toronto — say in a paper published Thursday in the journal Science.
They suggest China may need to rethink the management of live poultry markets, especially in urban areas, to prevent the virus from fully adapting to spread among people.
The outbreak, which appears to have come under control — albeit perhaps temporarily — has so far resulted in 131 confirmed human infections and 36 deaths.
There hasn't been a new case reported since May 8. Still, flu experts warn that it is too soon to say that the virus is gone. Warming temperatures in China and the closure of live bird markets may have temporarily stopped human infections, but they could resume in the fall.
Malik Peiris, one of the authors of the study, said the research is part of the process of assessing the risk posed by this virus, which first came to the attention of the international community at the beginning of April.
The researchers experimentally infected ferrets, which are frequently used as a stand-in for people in flu studies. They then placed uninfected ferrets into cages with the infected animals. The virus easily passed from the sick to the healthy animals.
They also put healthy animals in cages near infected ferrets, to see if the virus could spread the way it does between people — by viruses transmitted through coughing and sneezing. Only one of three ferrets in adjacent cages got sick.
The results essentially reflect what has been seen in humans. The virus clearly can transmit to people, but doesn't appear to spread easily from one person to the next at this point. There have been small clusters of cases, though, and they may suggest that when people are in close contact with an infected person they can contract the virus.
Flu experts don't take any comfort from the fact that the airborne spread appeared to be inefficient in this study. That's because with other bird flu viruses, it wouldn't have been seen at all.
"You do this (experiment) with H5N1 with a virus that comes out of humans or poultry, it won't transmit. And you do this with any other avian virus, it won't transmit. But this one does," said Ron Fouchier, a Dutch virologist who specializes in studying the mutations bird flu viruses need to be able to spread by the airborne route among mammals.
"This virus is already well on its way. ... It is poorly transmissible, but it can transmit. So there are reasons to date to take this virus seriously."
Fouchier, who is with Erasmus Medical Centre in Rotterdam, the Netherlands, said science doesn't currently know what other mutations the H7N9 virus would need to become full humanized.
Peiris concurred that this virus needs to be considered a real risk.
He also noted, though, that the infection in ferrets was not severe, which supports the possibility that there may be mild human cases that aren't being detected. Modelling work by one of Peiris's colleagues has suggested that for every serious infection being seen there may be between two and five milder cases being missed, probably in younger people.
"The fact that the (illness in) ferrets is relatively mild then raises the question as to what the total denomination of these infections are," said Peiris, who chairs the microbiology department at the University of Hong Kong.
In another interesting aspect of the study, the researchers experimentally infected pigs. Pigs are susceptible to flu viruses. And because they can be infected with both bird and human flu strains, they are thought to play a key role in the formation of new hybrid strains.
The researchers found pigs became infected with H7N9, but didn't pass the virus from infected animal to healthy animal.
Peiris said that means pigs probably are not currently part of the interplay between this virus and people.
"So I think what these data would suggest is that it is pretty unlikely that this virus came from pigs. And by implication the dead pigs in Shanghai were probably not related to this story," said Peiris.
Around the time H7N9 was first spotted, there were alarming reports of thousands of dead pigs found floating down a river near Shanghai. People were concerned the events were linked, but testing on some of the pigs came back negative.Suggest a correction