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Much depends on whether the virus erupts as school resumes or waits until the vaccine arrives in October.

By LAURAN NEERGAARD, The Associated Press August 31, 2009

WASHINGTON — The alarm sounded with two sneezy children in California in April. Just five months later, the never-before-seen swine flu has become the world's dominant strain of influenza, and it's putting a shockingly younger face on flu.

So get ready. With flu's favorite chilly weather fast approaching, we're going to be a sick nation this fall. The big unknown is how sick. One in five people infected, or a worst-case scenario – half the population? The usual 36,000 deaths from flu, or tens of thousands more?

The World Health Organization predicts that within two years, nearly one-third of the world's population will have caught it.

"What we know is, it's brand-new and no one really has an immunity to this disease," said Health and Human Services Secretary Kathleen Sebelius.

A lot depends on whether the swine flu that simmered all summer erupts immediately as students crowd back into schools and colleges – or holds off until millions of vaccine doses start arriving in mid-October.

This week, U.S. researchers start blood tests to answer a key question: How many doses of H1N1 vaccine does it take to protect? The answer will determine whether many people need to line up for two flu shots – one against swine flu and one against the regular flu – or three.

The hopeful news: Even with no vaccine, winter is ending in the Southern Hemisphere without as much havoc as doctors had feared, a heavy season but not an overwhelming one. The strain that doctors call the 2009 H1N1 flu isn't any deadlier than typical winter flu so far. Most people recover without treatment; many become only mildly ill.

Importantly, genetic tracking shows no sign that the virus is mutating into a harsher strain.

Regular flu, sadly, kills mostly grandparents. The real shock of swine flu is that infections are 20 times more common in the 5- to 24-year-old age group than in people over 65. That older generation appears to have some resistance, probably because of exposure decades ago to viruses similar to the new one.

Worldwide, swine flu is killing mostly people in their 20s, 30s and 40s. Especially at risk are pregnant women. So are people with chronic conditions such as asthma, diabetes and heart disease. Some countries report more deaths among the obese.

Still, some of those who have died had no obvious health risks.

"People who argue we're seeing the same death rates miss the point – they're in young adults. To me, that shouldn't happen," said one infectious-disease specialist, Dr. Richard P. Wenzel of Virginia Commonwealth University. He spent the past few months visiting South American hospitals to help gauge what the Northern Hemisphere is about to face.

Children and young people, however, are the flu's prime spreaders. Already, elementary schools and colleges are reporting small clusters of sick students. For parents, the big fear is how many children will die.

Regular flu kills 80 to 100 U.S. children every winter, and the Centers for Disease Control and Prevention has reports of about 36 child deaths from H1N1.

Even if the risk of death is no higher than in a normal year, the sheer volume of ill youngsters means "a greater-than-expected number of deaths in children is likely," said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases. "As a society, that's something that's much harder for us."

NEW STRAIN MAKES UP MOST CASES

Swine flu quietly sickened hundreds in Mexico before U.S. researchers stumbled across two children in San Diego with the same mystery illness. Mexico closed schools and restaurants, and barred spectators from soccer games; China quarantined tourists. But there was no stopping H1N1 from becoming the first pandemic in 41 years.

Well over 1 million Americans caught swine flu in the spring and summer months when influenza...