New Delhi, July 4: The new H1N1 virus has not spread like wildfire despite opportunities because its genetic makeup allows only inefficient handshakes with the gateways to the human respiratory tract, scientists have said.

Scientists who examined genes that play a key role in the transmission of influenza viruses have discovered that the H1N1 virus appears to spread from person to person less effectively than other flu viruses.

Indian biologist Ram Sasisekharan at the Massachusetts Institute of Technology (MIT) and his colleagues at the US Centres for Disease Control (CDC) have found that a surface protein on the H1N1 virus binds inefficiently with receptors — gateway molecules — on the cells of the human respiratory tract through which the virus infects people. The scientists reported their findings yesterday in the US journal Science.

“While the (H1N1) virus is able to bind to human receptors, it (the virus’s ability to do so) clearly appears to be restricted,” Sasisekharan said. This may explain why H1N1 hasn’t spread as efficiently as other flu viruses.

The researchers have cautioned that flu viruses can mutate fast and there will be cause for greater concern if H1N1 too changes its genetic makeup in the coming months.

“It appears we have dodged the bullet at the present time, but it is important to remain vigilant and to pay close attention to how this virus evolves,” Sasisekharan said. “The threat is not over yet.”

Although the virus, which surfaced in Mexico earlier this year, has infected over 70,000 people in 120 countries, scientists believe it could have spread faster, especially given people’s domestic and international travel patterns today.

Last week, a Pune man infected with H1N1 travelled by plane and train to Lucknow, sparking fears among health officials that clusters of infection might emerge from local spread of the virus.

India’s flu surveillance network has detected 128 cases of H1N1, all among people who have travelled abroad or their close relatives. There is no evidence for clusters so far.

The new study by the MIT-CDC team showing that H1N1 viruses bind less effectively with the umbrella-shaped receptor molecules in the human respiratory tract might explain the absence of local clusters.

The researchers also examined a second protein, called PB2, in H1N1 that appears to impair the ability of the virus to spread fast. The virus does not have a specific version of PB2 that helps efficient transmission.

The presence of an amino acid molecule called lysine at a particular position in this protein supports efficient influenza virus replication at low temperature. All three of the 20th-century flu pandemics were caused by viruses that had mainly lysine at this position. But the new H1N1 virus has another amino acid, called glutamic acid, at this position, which appears to impair its ability to spread.