Immunologists expressed concern Friday about the "dangerous" work of scientists in China who created a hybrid bird flu virus that can spread in the air between guinea pigs, and now lives in a lab freezer.
The team from the Chinese Academy of Agricultural Sciences and Gansu Agricultural University wrote in the journal Science they had created a new virus by mixing genes from H5N1 "bird flu" and H1N1 "swine flu."
H5N1, transmitted to people by birds, is fatal in about 60 percent of cases, but does not transmit between humans -- a characteristic that has prevented a pandemic so far.
Some argue that hybrid studies like these shed light on how the virus could mutate in nature to cause a human epidemic, and may help us prepare.
Since 2003, H5N1 has infected 628 people, killing 374, according to the World Health Organization.
H1N1, which erupted in Mexico, is highly transmissible and infected a fifth of the world's population in a 2009-10 pandemic, but is about as lethal as ordinary flu.
The new mutant virus was easily transmitted between guinea pigs through respiratory droplets -- which the Chinese team said proved the deadly H5N1 virus may need but a simple genetic mutation to "acquire mammalian transmissibility."
Flu hybrids can arise in nature when two virus strains infect the same cell and exchange genes in a process known as reassortment, but there is no evidence that H1N1 and H5N1 have done so yet.
Some observers fear that science is putting mankind at risk by preemptively creating such mutants.
"These are manmade viruses, they have never been made in Nature. They are now sitting in a freezer," virology professor Simon Wain-Hobson of France's Pasteur Institute told AFP.
He pointed to a laboratory leak of foot and mouth, a cattle disease, which caused an outbreak in Britain six years ago.
It was unclear how the flu hybrid, which is not deadly in guinea pigs, would affect people -- but Wain-Hobson warned: "These could be pandemic viruses.
"That is, if there was ever an error of they got out or there was a leak or whatever, this could infect people and cause anywhere between 100,000 and 100 million deaths."
In January, scientists in the United States and the Netherlands resumed controversial research into their own hybrid flu viruses after taking a year-long break to allay fears of the bug escaping the lab or falling into terrorist hands.
Their creation was able to jump between ferrets, considered a good research model for human disease spread.
The US-Dutch teams cited a "public health responsibility" to resume the work, halted after a public outcry and global safety probes.
Jeremy Farrar, director of the Oxford University Clinical Research Unit in Vietnam, told Nature News the new study showed that H5N1 continued to pose a very real threat.
"I do believe such research is critical to our understanding of influenza. But such work, anywhere in the world, needs to be tightly regulated and conducted in the most secure facilities, which are registered and certified to a common international standard," he said.