Return of Swine Flu: What's Ahead for Americans?

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 -- 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," Health and Human Services Secretary Kathleen Sebelius says.

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.

Only this week do U.S. researchers start blood tests to answer a critical question: How many doses of swine flu 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 that started early 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, careful genetic tracking shows no sign yet that the virus is mutating into a harsher strain.

We're used to regular flu that, sadly, kills mostly grandparents. But 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.