As the weather turns cooler and kids across the nation begin filling classrooms, U.S. health officials are bracing for a return to more than just the books. It’s back to flu season, and worries about the H1N1 virus—also known as swine flu—promise to keep healthcare workers on their toes.

The latest government estimates forecast up to 1.8 million people hospitalized and between 30,000 to 90,000 swine flu deaths, according to an H1N1 report released Monday by the President’s Council of Advisors on Science and Technology (PCAST). The report, compiled by a 21-member panel of PCAST-chosen scientists, indicates more than half the U.S. population could be infected.

“This is going to be fairly serious,” said Harold E. Varmus, co-chair of the 21-member PCAST council, in a Washingon Post interview. “It's going to stress every aspect of our health system.”

Or it might not. The report takes an aggressive stance on preparedness, including recommendations to clearly define guidelines for antiviral use, hasten decision making on the use of intravenous antivirals, and step up vaccine availability by bottling medicine before dosage studies are complete. But it also notes the scenario used to create the estimates is merely plausible and a milder outbreak is possible.

The estimates were based on swine flu behavior earlier this year and previous pandemics. Swine flu isn’t thought to be deadlier than regular flu (and some infectious disease experts believe it will remain that way), but many are worried it’s impact will be greater. Those fears are due mostly to a lack of immunity in the current population and the fact that its preferred victims—school-aged children and young adults—are already consummate spreaders of germs and viruses, according to a related Post article.

If the greater-spread scenario does manifest, there could be problems. Vaccine development hasn’t gone as planned, with only about a third of the amount originally projected likely to be ready by October when H1N1 cases are expected to peak, according to a third Washington Post article on vaccine production. Overuse of antivirals to treat flu symptoms, which has already lead to several cases of Tamiflu-resistant strains of H1N1, is also a concern, according to the Los Angeles Times.

For those vaccines that are produced in time, questions about dosage and delivery, the need for boosters and adjuvants, and safety remain a concern.

"We are continuing oversight on the quality and safety of the vaccine being produced, and the production process itself," the Food and Drug Administration’s Jesse Goodman told the Post. "That's going well so far, but our oversight is continuing."

Those assurances haven’t been enough to keep the specter of a failed 1976 vaccination program from rearing its head. That program, which vaccinated 40 million against swine flu, is blamed for causing an above-average amount of Guillain-Barré Syndrome, which causes paralysis, according to the Post.

In the face of all the swine flu uncertainty, prevention is likely to be the best health investment, according to a White House statement.

“An overarching message of the new report is that through their behavior, individuals can have a potentially big impact on the flu season’s severity,” it stated. “Frequent hand-washing and staying home from school or work when sick will be crucial.”