The study, led by doctors Ian A. Wilson and James C. Paulson, of The Scripps Research Institute, suggests that the virus' current form is unlikely to cause a pandemic, the agency said in a news release.
The study, published Dec. 5 in Science, was supported by NIAID, part of the National Institutes of Health, and other organizations, the release stated.
As of Nov. 6, 139 confirmed human cases of avian H7N9 influenza, including 45 deaths, have been reported by the World Health Organization.
Most of the cases have been linked to exposure to infected poultry, but in some cases, limited human-to-human transmission may have occurred, the agency said.
In the study, the TSRI scientists examined the three-dimensional structures of the hemagglutinin (HA) protein on the surface of the virus and its interaction with the human influenza receptor—the molecule on the surface of human cells that HA binds to before entering the cell and causing infection.
Previous research had shown that compared to influenza viruses adapted to spread easily among birds, viruses adapted to humans generally have different amino acids (protein components) at the HA site that recognize and bind to the human receptor, the release added.
Recent studies have shown that certain H7N9 viruses had acquired mutations that might make them more adapted to humans, the release said.
However, using X-ray crystallography to study the HA and receptor structures with unprecedented accuracy, the TSRI researchers demonstrated that the HA in avian H7N9 influenza most closely resembles that of viruses that spread easily among birds, yet only weakly attaches to human influenza receptors.
The Centers for Disease Control and Prevention reported an outbreak of human infections with the virus in China on April 1, 2013.
The virus was detected in poultry in China as well. Most human infections are believed to have occurred after exposure to infected poultry or contaminated environments, the CDC said.
Although it is not impossible that the H7N9 virus could eventually become transmissible from person to person, it would need to undergo multiple other mutations to do so, the study authors wrote.
NIAID's news about the study comes in the wake of Hong Kong’s Center of Health Protection's announcement Dec. 6 that a second person, an elderly Chinese man, had contracted the H7N9 bird flu virus disease necessitating the quarantine of 19 people as a precautionary measure.