WASHINGTON, March 11 (Xinhua) -- Two flu strains that recently began causing sporadic infections among people in Chinese mainland and Taiwan, known as H10N8 and H6N1, are still not able to infect humans easily, U.S. researchers said Wednesday.

Researchers from the Scripps Research Institute (TSRI) analyzed a key protein from the two strains and found they have not acquired changes that would cause a much-feared pandemic.

However, given the versatility that these bird flu viruses have in attaching to bird cells, they "can probably mutate in different ways to jump to humans," said Ian Wilson, professor of structural biology and chair of TSRI´s Department of Integrative Structural and Computational Biology. "So we shouldn´t be complacent about our ability to predict the viral changes required to get a pandemic."

The new research, from Wilson´s laboratory and the laboratory of James Paulson, acting president and CEO of TSRI, was reported in two papers in the U.S. journal Cell Host & Microbe.

One paper focused on an H10N8 bird flu virus that infected and killed a 73-year-old Chinese woman from Jiangxi Province in late 2013 and reportedly caused at least two other deaths in that province since then.

Using the genetic sequence of the virus obtained by Chinese scientists, the TSRI team produced copies in cell culture of its hemagglutinin (HA) protein, which flu viruses use to infect host cells.

Paulson´s laboratory tested the ability of the human-infecting H10N8 HA protein to bind to lab-grown versions of "flu virus receptors" and found that it binds well to the receptors found on bird cells, but negligibly to those found on human cells.

Wilson´s laboratory used X-ray crystallography to determine the atomic-scale structure of the H10N8 HA protein, revealing how the protein binds to avian receptors and why it cannot bind well to human receptors.

"This H10N8 HA protein hasn´t acquired the structural changes that would allow the virus to be transmitted efficiently from human to human," said co-first author Heng Zhang, a research associate in the Wilson laboratory.

The other paper, of the HA protein from an H6N1 flu virus that caused nonfatal illness in a 20-year-old Taiwanese woman in June 2013, came to essentially the same conclusions: The viral HA had changed somewhat compared to the usual HA from bird-isolated H6N1, but still remained adapted for binding to avian, not human, receptors.

The research also showed that at the atomic scale these new bird flu HAs bind to host-cell receptors in ways not observed in studies of other bird flu viruses, implying that the mutations required for easily infecting humans may be different for different strains and thus hard to know in advance.