The Obama administration will spend an additional $2.7 billion to buy swine flu drugs and vaccines, just days after White House science advisers called the pandemic a serious threat to our nation.

The money is on top of $1.8 billion the administration earmarked in July for tackling the virus, which is expected to worsen in the Northern Hemisphere's autumn as students return to school.

In a letter to House of Representatives Speaker Nancy Pelosi on Tuesday, President Barack Obama said there was still uncertainty about the potential severity of any new outbreak.

The President's Council of Advisors on Science and Technology said in a report last week that the virus, although moderate, could infect 30 percent to 50 percent of the U.S. population this year, put 1.8 million people into the hospital and kill anywhere between 30,000 and 90,000 people.

Obama said the $2.7 billion would go toward buying new vaccines, antiviral drugs and preparing for a vaccination campaign.

The money is part of $7.65 billion that the U.S. Congress has appropriated for the Health Department this year and does not represent new funds on top of what has already been approved.