A top expert has raised fears that the swine flu is unlikely to become more virulent as it spreads through the northern hemisphere this winter, but could re-emerge a year later in a more deadly form.
"We should get through the winter relatively easily, I don't think the virus will mutate before then," said John Oxford, a professor of virology at Britain's St Bartholomew's and the Royal London Hospital.
"There will be more people in hospital and more deaths, but essentially it will be the same virus we have experienced in the summer, just more of it," he told AFP in an interview.
After winter has passed, however, the pressures of natural selection could favour the emergence of more deadly strains of the A(H1N1) virus, Oxford explained.
"For the moment, the virus is running around the world finding lots of young people and infecting them. It is doing very nicely, thank you, why should it change?", he said by phone.
"But once the virus has infected about a third of the world's population -- which is what we expect -- it will find less 'susceptibles'. That is when mutants will have a selective advantage."
It would be a serious mistake to think that once the impending flu season is over, the danger will have passed, he added.
Oxford said he had just returned from Australia, where he met front-line doctors who were concerned about an emerging pattern in swine flu patients.
Whether they are people in high risk groups -- the obese, pregnant women, asthmatics -- or young adults with no underlying conditions, an alarming number of patients wind up in intensive care units.
"One minute they are OK in a hospital bed, the next minute they are in intensive care," he said.
There have been more than 100 confirmed deaths from the pandemic H1N1 strain in Australia, which is just emerging from the southern hemisphere winter.
Epidemiologists sifting through data from other countries have also found similar -- and disquieting -- patterns.
French epidemiologist Antoine Flahault reported a 100-fold increase, compared to seasonal flu, in the number of swine flu deaths in Mauritius and New Caledonia attributed directly to the virus itself rather than secondary bacterial infections or underlying conditions.
Many of those deaths were caused by acute respiratory disease syndrome (ARDS), which requires intensive-care treatment for an average of three weeks. Only 50 percent of ARDS patients survive.