The H7N9 bird flu virus has not yet acquired the changes needed to infect humans easily but it would not be wise to dismiss its potential risk, according to a U. S. study published in the journal Science Thursday. In contrast to some initial studies that had suggested that H7N9 poses an imminent risk of a global pandemic, the new research found, based on analyses of virus samples from the outbreak in China earlier this year, that H7N9 is still mainly adapted for infecting birds. "Luckily, H7N9 viruses just don't yet seem well adapted for binding to human receptors," said senior author Ian Wilson, professor at The Scripps Research Institute (TSRI), a California- based medical research facility, in a statement. "Because publications to date have implied that H7N9 has adapted to human receptors, we felt we should make a clear statement about this," added James Paulson, chair of TSRI's Department of Cell and Molecular Biology. China had reported 136 human infection cases with 45 fatalities by the end of October. The big question was whether the virus was capable of spreading only in a limited, sporadic way from birds to humans or if it had truly "jumped the species barrier." Paulson and Wilson evaluated H7N9's ability to bind the sialylated sugar receptors to which flu viruses normally attach on host cells. The researchers tested the ability of the virus's hemagglutinin (HA) protein, to bind to different human and avian receptor variants and found that the isolate from a Shanghai patient still has a strong preference for avian-type receptors and binds human- type receptor variants only weakly. They also performed X-ray crystallography studies of the patient's HA protein bound to several avian- and human-type receptors, which showed loose contacts that the patient's HA makes with human-type receptors, in contrast to the snug couplings it makes with certain avian-type receptors. "Our research reveals that the receptor binding specificity of H7N9 virus is similar to known avian viruses, with preferences to avian-like receptors," Xu Rui, TSRI researcher and first author of the study, told Xinhua. "It suggests that the virus in its current form is fit for efficient transmission in birds but not in humans. " Despite hints that it had begun to adapt to human hosts rather than its natural bird ones, H7N9 did not appear to pose an imminent threat of a human pandemic, the researchers said. "It is impossible to predict how it will take or even whether it will eventually adapt for human transmission. It is important to continue to observe this virus to see if it undergoes any changes that would improve its transmission in humans," Xu said. " We believe it is a high-risk virus that requires close watching, like the previously known avian flu H5N1."