* New virus could still mutate into more virulent form
* Flu pandemic could be expected to circle globe 3 times
* Antiviral drugs recommended only for high-risk patients
* Official global tally is 5,251 cases, including 61 deaths
(Adds WHO flu expert on antivirals)
By Stephanie Nebehay and Laura MacInnis
GENEVA, May 12 (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.
Aspirin should not be used because of a risk of liver damage, she added.
LESS SEVERE IN EUROPE
The Swiss drugmaker Roche said earlier on Tuesday it would donate 5.65 million packs of its Tamiflu to the WHO to replenish stockpiles deployed against the H1N1 outbreak and said it was ramping up production of the drug.
European countries on alert for the flu, whose emergence caused the WHO to declare a global pandemic is "imminent," have been prescribing antivirals to treat those infected and reduce the risk they will spread it to others around them.
Shindo allowed that such aggressive therapy may have helped reduce the severity of the disease outside its epicentre Mexico, where 56 people have died from infection. "It could be the reason why we are seeing less severe cases in Europe," she said.
H1N1 flu has also killed three people in the United States and one each in Canada and Costa Rica, according to WHO figures.
Few of the victims appear to have been completely healthy before catching and succumbing to the flu, according to Shindo, who said the new WHO guidance would reflect findings that the strain was most dangerous to those with chronic health problems.
Governments will likely adopt a variety of strategies about who should get antivirals, with the WHO guidance as a reference point. "The countries have to make their prioritisation," Shindo said. "There will be discussions about how to use it."
According to the United Nations agency's latest tally, some 5,251 people have been infected with H1N1 flu in 30 countries.
The virus has caused mild flu-like symptoms such as nausea, fever, and diarrhoea in most patients, and some people infected with it have even been asymptomatic.
But flu strains mutate frequently and unpredictably and "the emergence of an inherently more virulent virus during the course of a pandemic can never be ruled out," the WHO said.
"The overall severity of a pandemic is further influenced by the tendency of pandemics to encircle the globe in at least two, sometimes three, waves," it said. "For many reasons, the severity of subsequent waves can differ dramatically in some or even most countries."
The 1918 influenza pandemic -- which killed tens of millions of people -- began mild and returned within six months in a much more lethal form. The 1968 pandemic began relatively mild, with sporadic cases prior to the first wave, and remained mild in its second wave in most, but not all countries.