'They are not a prediction, but they are a possibility. If it turned out to affect a lot more adults, the severity would be a lot worse,' Lipsitch said.

While the seasonal flu is associated with 30,000 to 40,000 deaths and 200,000 hospitalisations each year, the lack of immunity to the swine flu virus probably will lead to many more people becoming infected, sick -- and possibly to 30,000 to 90,000 deaths, the Washington Post reported.

It was estimated that the outbreak could peak in mid-October, so the panel urged expediting the availability of an anti-flu vaccine.

Health and Human Services Secretary Kathleen Sebelius said: 'This isn't the flu that we're used to.

'The 2009 H1N1 virus will cause a more serious threat this fall. We won't know until we're in the middle of the flu season how serious the threat is, but because it's a new strain, it's likely to infect more people than usual.'

The pandemic has caused significant disruptions and economic damage in parts of the Southern Hemisphere and has contributed to the deaths of 1,799 people in 168 countries, including at least 522 in the US.

A second wave of infection is expected to begin within weeks in the Northern Hemisphere as schools reopen and cooler weather returns.