Swine flu is likely to spread around the world in the next few months and infect one-third of the global population, according to the first detailed analysis of the spread of the virus published by British scientists.
The study by researchers at London's Imperial College, published in the journal Science, found that swine flu has "full pandemic potential", spreading easily from person to person and infecting around one in three of those who come into contact with it.
But the research's author, Professor Neil Ferguson, said it was too early to say whether the virus will cause deaths on a massive scale, or prove little more lethal than normal seasonal flu.
Its full impact on the UK is not likely to be known until the annual flu season in the autumn and winter, when a "really major epidemic" can be expected in the northern hemisphere.
Prof Ferguson, who sits on the World Health Organisation's emergency committee for the outbreak, said the international community should decide this week whether to switch vaccine-production capacity away from seasonal flu to concentrate on swine flu.
"This virus really does have full pandemic potential," he told the BBC Radio 4 Today programme. "It is likely to spread around the world in the next six to nine months and when it does so it will affect about one-third of the world's population.
"To put that into context, normal seasonal flu every year probably affects around 10 per cent of the world's population every year, so we are heading for a flu season which is perhaps three times worse than usual – not allowing for whether this virus is more severe than normal seasonal flu viruses."
Prof Ferguson declined to put an estimate on the number of deaths which may result from the swine flu pandemic.
"We have some assessment, but the uncertainty is still quite broad," he said. "We can say it is not going to be as catastrophic as the Spanish flu pandemic of 1918 – it is milder than that.
"But it is still possible it could be (like) 1957 – where about three to four out of 1,000 people who were infected died and overall about 3-4 million people died that year because of the pandemic worldwide – or it could be even milder than that, like the 1968 pandemic which was barely worse than a normal seasonal flu year.
"I am not predicting 3-4 million (deaths). That was what happened in 1957. The world is a very different place today. There are more people in the world, but there is also a much better healthcare system. We have drugs and vaccines, particularly in developed countries, which should markedly reduce the burdens of the disease."