The World Health Organisation has warned that there is no guarantee that a pandemic strain of the H5N1 virus will be less deadly to people. The report, published this week, contains the findings of a meeting of flu experts held last month.

The method of change will determine how deadly the virus is, the report says. If the virus changes by exchanging genes with a human flu virus, a process called reassortment, a pandemic might not be too deadly. However, straightforward adaptive mutation would leave the virulence of the disease unchanged, the scientists warned.

It also cautioned against regarding the anti-viral drug Tamiflu as a magic bullet, highlighting that some H5N1 viruses seem to already be naturally resistant to the effects of the drug.

The report comes less than two weeks after the Centre for Ecology and Hydrology (CEH) in the UK issued a warning about the possible environmental effects of large scale use of Tamiflu.

Dr Andrew Singer of the CEH said: "An antiviral drug has never been widely used before, so we need to determine what might happen. During a flu pandemic, millions of people will all take Tamiflu at the same time. Over just eight or nine weeks, massive amounts of the drug will be expelled in sewage and find its way into the rivers. It could have huge effects on the fish and other wildlife."

A build up of the drug in rivers could cause the avian version of the virus to become resistant to it, which could lead to a new, resistant strain emerging in humans, Singer argues.

He is calling for research into ways to safely break the anti-viral down once it is in the sewage system but before it reaches the public waterways. ®