The new A/H1N1 strain of flu has a form of surface protein that binds inefficiently to receptors found in the human respiratory tract, which make it spread from person to person less effectively than other flu viruses, scientists said.

A team from Massachusetts Institute of Technology (MIT) and the U.S. Centers for Disease Control and Prevention (CDC) reported the discovery Thursday in the online edition of Science.

"While the virus is able to bind human receptors, it clearly appears to be restricted," says Ram Sasisekharan, director of the Harvard-MIT Division of Health Sciences and Technology (HST) and the lead MIT author of the paper. Sasisekharan and his laboratory co-workers have been actively investigating influenza viruses.

That restricted, or weak, binding, along with a genetic variation in an H1N1 polymerase enzyme, which MIT researchers first reported three weeks ago in Nature Biotechnology, explains why the virus has not spread as efficiently as seasonal flu, says Sasisekharan.

Sasisekharan and CDC senior microbiologist Terrence Tumpey have previously shown that a flu virus's ability to infect humans depends on whether its hemagglutinin protein can bind to a specific type of receptor on the surface of human respiratory cells.

In the new Science paper, Sasisekharan, Tumpey and colleagues compared the new H1N1 strain to several seasonal flu strains, including some milder H1N1 strains, and to the virus that caused the 1918 flu pandemic. They found that the new strain, as expected, is able to bind to the predominant receptors in the human respiratory tract, known as umbrella-shaped alpha 2-6 glycan receptors.

The researchers also found that the new H1N1 strain spreads inefficiently in ferrets, which accurately mimics human influenza disease including how it spreads or transmits in humans.

When the ferrets were in close contact with each other, they were exposed to enough virus particles that infection spread easily. However, when ferrets were kept separate and the virus could spread only through airborne respiratory droplets, the illness spread much less effectively.

This is consistent with the transmission of this virus seen in humans so far, says Sasisekharan. Most outbreaks have occurred in limited clusters, sometimes within a family or a school but not spread much further.

However, flu viruses are known to mutate rapidly, so there is cause for concern if H1N1 undergoes mutations that improve its binding affinity.

"We need to pay careful attention to the evolution of this virus," says Sasisekharan.

The World Health Organization announced Wednesday a total of 77,201 people have been infected with the A/H1N1 flu virus worldwide, as 332 among them died.

Source: Xinhua