In nature, some strains of the influenza virus are highly lethal while others jump easily from person to person. What public health officials fear most is a hybrid that combines the lethality of one with the transmissibility of the other, creating a deadly global pandemic.

Now a team of Chinese scientists has investigated that in their lab by creating a new hybrid virus. They combined H5N1 avian influenza, which is highly lethal but doesn't transmit easily between people, with the highly contagious H1N1 swine flu strain responsible for infecting tens of millions of people in 2009.

The new hybrid virus passed easily between guinea pigs, which are used to study how flu infects mammals. Molecular changes in the virus may provide clues of what to look for in circulating H5N1 strains, perhaps allowing scientists to anticipate when viruses will more easily infect humans.

"Mammalian-transmissible H5N1 viruses can be generated in nature," said virologist Chen Hualan at China's Harbin Veterinary Research Institute, who led the research team. "High attention should be paid during routine influenza surveillance to monitor such high risk H5N1 hybrid viruses in nature."

The experiments, described May 2 in Science, reflect a controversial approach to studying influenza: attempting to create strains in a lab that would, if accidentally released or used for nefarious purposes, pose a potentially global health threat.

Some scientists think the risks don't outweigh the benefits, and that institutional safeguards don't sufficiently reduce chances of accidents. Public unease with such experiments resulted in a year-long moratorium on the research.

According to microbiologist Richard Ebright of Rutgers University, a prominent critic of the work, the new study shows little that wasn't revealed by an H5N1 experiment published in March by one of the research teams that originally sparked the controversy.

That study, led by virologist Ron Fouchier of Erasmus University in the Netherlands, also showed how H5N1 could become more transmissible in mammals.

"The sole major difference is the use of guinea pigs in this paper and ferrets in that paper," said Ebright. "In my assessment, neither paper contains substantive new information that justifies the risks posed by the research."

Hualan's next research focus is to study the array of genetic changes that allowed his H5N1 strains to infect guinea pigs, perhaps understanding the specific molecular and physiological mechanisms that help particles of flu virus latch onto respiratory cells.

That might give researchers clues about how to interfere with that process, or at least a heads-up on what type of changes to look for in flu strains evolving now in the wild.

"The important thing is that insights from such studies will benefit disease control and prevention," Hualan said. Both she and Ebright agreed, however, that mutations seen in the laboratory may be different from the evolutionary paths taken by influenza in the wild.

Citation: "H5N1 Hybrid Viruses Bearing 2009/H1N1 Virus Genes Transmit in Guinea Pigs by Respiratory Droplet." By Ying Zhang, Qianyi Zhang, Huihui Kong, Yongping Jiang, Yuwei Gao, Guohua Deng, Jianzhong Shi, Guobin Tian, Liling Liu, Jinxiong Liu, Yuntao Guan, Zhigao Bu, Hualan Chen. Science, Vol. 340 No. 6132, 3 May 2013.

Correction 5/2: The article originally stated that the new study resembled one published in June 2012 by Ron Fouchier and colleagues. The reference was incorrect; the related study was published in March 2013.