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H7N9 Bird Flu Is Poorly Adapted for Infecting Humans

Pandemic is unlikely, scientists say (December 6)

Avian influenza virus H7N9, which killed several dozen people in China earlier this year, has not yet acquired the changes needed to infect humans easily, according to a new study by scientists at The Scripps Research Institute (TSRI).

In contrast to some initial studies suggesting that H7N9 poses an imminent risk of a global pandemic, the new research found, based on analyses of virus samples from the Chinese outbreak, that H7N9 is still mainly adapted for infecting birds, not humans.

The report appears in the Dec. 6 issue of Science.

H7N9 flu viruses infect birds, apparently causing them few or no symptoms. Until this year, these strains had never been reported in humans. However, starting in February in two urban areas of eastern China, dozens of people began to come down with H7N9 flu. Most became severely ill. By the end of May, when the outbreak had mostly subsided, there were 132 laboratory-confirmed human cases and 37 deaths — a case–fatality rate of nearly 30%.

One group of investigators at TSRI tested the ability of the virus’ hemagglutinin (HA) protein to bind to different human and avian receptor variants. These tests showed that the isolate tested (A/Shanghai/2/2013 [Sh2]) still has a strong preference for avian-type receptors and binds human-type receptor variants only weakly.

Another group of researchers performed x-ray crystallography studies of the Sh2 HA protein bound to several avian- and human-type receptors. The new data highlighted the looseness of the contacts that Sh2 HA makes with human-type receptors, in contrast to the snug couplings it makes with certain avian-type receptors.

Thus, despite hints that H7N9 had begun to adapt to human hosts rather than its natural bird hosts, the virus does not appear to pose an imminent threat of a human pandemic, the scientists say.

Sources: Science; December 6, 2013; and TSRI; December 5, 2013.

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