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 Organisation.

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."

Wain-Hobson and others fear the risk may far outweight the scientific value of the research.

The findings held little value for finding a vaccine or treatment that would take years to develop -- probably long after an outbreak, they argue.

"The record of containment in the highest containment laboratories is not good. There have been repeated leaks," said Robert May, a former president of Britain's Royal Society of science.

"You do not do these things unless there is some call of extreme emergency," he said. "We are encountering a real and present danger with extremely dubious benefits to the public."

Virologist John Oxford from the Queen Mary University of London, however, said the experiment was a valuable wakeup call.

It showed that the two viruses, both still infecting people around the world, can swap genes.

"Mathematics will tell you that sooner or later a person will get co-infected," he said -- possibly leading to a hybrid virus "that will start spreading".

"We need to get ourselves reorganises, relook our pandemic plans and make sure we have H5N1 vaccine stockpiles," Oxford said.

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.