The virus could cause symptoms in 60 million to 120 million people, more than half of whom might seek medical attention, the President’s Council of Advisors on Science and Technology estimated in an 86-page report to the White House assessing the government’s response to the first influenza pandemic in 41 years.

Although most of the cases probably would be mild, up to 300,000 people could require intensive care, which could tie up all those beds in some parts of the country at the peak of the outbreak, the council said.

“This is going to be fairly serious,” said Harold E. Varmus of Memorial Sloan-Kettering Cancer Center in New York, co-chair of the 21-member council. “It’s going to stress every aspect of our health system.”

The estimates mark the first time experts have released specific calculations about the possible U.S. impact of the pandemic. The “plausible scenario” is based on previous pandemics and how the swine flu behaved in the United States this spring and during the Southern Hemisphere’s winter over the past few months, said Marc Lipsitch of the Harvard School of Public Health, who helped prepare the estimate.

“They are not a prediction, but they are a possibility,” he said in a telephone interview, noting that the estimates are based on various assumptions, including that the virus will not mutate into a more dangerous form or infect more older people.

“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 hospitalizations 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 council said. And while most deaths during a typical flu season occur in the elderly, swine flu is more likely to kill children and young adults, the panel said.

Lipsitch stressed that the outbreak could turn out to be milder, too. The primary purpose of the estimates was to help guide planning to protect the public. For example, it was estimated that the outbreak could peak in mid-October, so the panel urged expediting the availability of a vaccine.

In addition, the panel recommended clarifying how antiviral drugs should be used to fight the pandemic, speeding a decision about whether to approve intravenous antivirals in case they are needed, designating someone at the White House to coordinate the nation’s response to the virus, and improving the system for tracking the spread of the new virus.

Swine flu virus, or H1N1, emerged last spring in Mexico and quickly spread to the United States and around the world. Although far less dangerous than initially feared, the virus has sickened children and young adults more frequently than the typical seasonal flu.

“This isn’t the flu that we’re used to,” said Health and Human Services Secretary Kathleen Sebelius. “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 more than 1,799 people in at least 168 countries, including at least 522 in the United States. A second wave of infection is expected to begin within weeks in the Northern Hemisphere as schools reopen and cooler weather returns.

Overall, the panel praised the federal government’s response, which has included signing contracts to spend nearly $2 billion to buy at least 159 million doses of vaccine from five companies that are rushing to produce it. But the first batch is not expected to be available until mid-October, when the outbreak could peak.

“This potential mismatch in timing could significantly diminish the usefulness of vaccination for mitigating the epidemic and could place many at risk for serious disease,” the report states.

The report recommends that a portion of the vaccine be made available by mid-September for those at highest risk by asking the manufacturers to start filling vials with vaccine even though the studies to determine dosages and whether a booster will be necessary have not been completed.

Administration officials said they are already taking action on the panel’s recommendations. All five companies “have been asked to put their initially available vaccine in vials as soon as they are ready,” for example. “This will move forward, even while awaiting results of clinical studies to confirm expected dosing, to ensure the earliest possible availability of initial doses of vaccine.”

“This report is being read very carefully,” said John O. Brennan, White House deputy national security adviser for counterterrorism.