WHO says new virus could still mutate

By Stephanie Nebehay and Laura MacInnis

GENEVA (Reuters) - The new H1N1 flu virus could still mutate into a more virulent form and spark an influenza pandemic that could be expected to circle the globe up to three times, the World Health Organisation said on Tuesday.

The impact of any pandemic would vary, as a virus that causes only mild illness in countries with strong health systems can become "devastating" in those with weak health systems, shortages of drugs and poorly equipped hospitals, it said.

The new virus, commonly referred to as swine flu, "appears to be more contagious than seasonal influenza" and practically the whole world lacks immunity to it, the WHO said in a document entitled "Assessing the severity of an influenza pandemic."

Nikki Shindo of the WHO's global influenza programme said that 10 percent of those infected with the strain in Mexico and the United States needed to be admitted to hospital -- far more than the rate for seasonal flu, which kills up to 500,000 people a year.

"This is clearly different than what we see from seasonal influenza," Shindo told a news conference.

Despite this, she said most patients could recover from H1N1 with simple steps like hydration, and without any drug treatment, raising questions about the rush to stockpile and prescribe antivirals to treat the disease.

The WHO will soon issue new guidance recommending that Tamiflu and other antiviral drugs be used only for vulnerable patients such as pregnant women and people with other health problems such as diabetes and cardiovascular disease, she said.

"We will recommend to consider the use of antivirals for high-risk groups or the group of people at increased risk, depending on the availability," said Shindo, a medical officer who heads WHO's clinical team.