Bird flu viruses would have to make at least two simultaneous genetic mutations before they could be transmitted readily from human to human, according to research published in PLoS ONE.

The authors of the new study, from Imperial College London, the University of Reading and the University of North Carolina, USA, argue that it is very unlikely that two genetic mutations would occur at the same time. The study adds to our understanding of why avian influenza has not yet caused a pandemic.

Earlier this year, the Imperial researchers also showed that avian influenza viruses do not thrive in humans because, at 32 degrees Celsius, the temperature inside a person's nose is too low.

This study suggests that one reason why H5N1 has not yet caused a pandemic is that two genetic mutations would need to happen to the virus at the same time in order to enable it to infect the right cells and become transmissible. At present, H5 viruses can only infect one of the two main types of cell in the mouth and nose, a type of cell known as a ciliated cell. In order for H5 to transmit from human to human, it would need to be able to infect the other, non-ciliated type of cell as well.