GENEVA - 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 Organization 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 nearly the world's whole population lacks immunity to the new disease, the WHO said in a document issued overnight entitled Assessing the severity of an influenza pandemic.

It attempts to explain different aspects of severity, not just the pathogenicity of the virus but its impact on health and social systems, WHO spokesman Thomas Abraham told Reuters. It is in response to questions from the public and media.

The United Nations agency's pandemic alert level is at 5, its second-highest level on a scale of 1 to 6, he said.

This means that the virus shows no signs of sustained person-to-person spread outside North America.

Some 5,251 people have been infected in 30 countries, according to WHO's tally on Tuesday, which includes 61 deaths. Hardest hit are Mexico with 2,059 cases, including 56 deaths, and the United States with 2,600 cases including three deaths.

So far, with the exception of the deadly outbreaks in Mexico, the virus has tended to cause very mild illness in otherwise healthy people, it said.

But influenza viruses 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.

CIRCLING THE GLOBE IN WAVES

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, the WHO said.

For many reasons, the severity of subsequent waves can differ dramatically in some or even most countries, it said.

The 1918 influenza pandemic -- which killed tens of millions of people -- began mild and returned within six months in a much more lethal form, it said. 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, it said.

The new virus appears to be more easily spread than seasonal flu. Regular seasonal flu kills about 250,000 to 500,000 people annually with a fatality rate of less than 0.1 percent.

Seasonal flu has a so-called secondary attack rate -- the percentage of contacts who catch it from an infected person -- of between 5 and 15 percent, according to the WHO.

Current estimates of the secondary attack rate of H1N1 range from 22 to 33 percent, it said.

Outside Mexico, nearly all cases of illness and all deaths have been in people with underlying chronic conditions such as cardiovascular disease, asthma and diabetes, according to WHO which has consulted widely with experts in affected countries.

In Mexico and the United States -- the two largest and best documented outbreaks to date -- a younger age group has been affected than during seasonal epidemics of influenza, it said.

Though cases have been confirmed in all age groups, from infants to the elderly, the youth of patients with severe or lethal infections is a striking feature of these early outbreaks, it said.

WHO acting assistant director-general Keiji Fukuda said on Monday it was not possible to produce a scientific assessment of the severity of the outbreak, on the lines of a hurricane warning, because of the unpredictable nature of the disease and the fact that different people and different countries experience the flu in different ways.

Mexico Medicine