Lauran Neergaard, AP Medical Writer

WASHINGTON (AP) — There’s good news in the world's flu fight. One dose of the new swine flu vaccine looks strong enough to protect adults and can spark protection within 10 days of the shot, Australian and U.S. researchers said Thursday.

In a study of 240 adults, half younger than 50 and half over, one shot prompted the same kind of immune response indicating protection that is seen with regular flu vaccine. And a standard 15-microgram dose — not the double dose that also was tested — was enough. “It's really striking how incredibly similar this is to every other study of a seasonal flu vaccine I've ever seen,” said Dr. John Treanor, a flu specialist at the University of Rochester who examined the data.

Australian shot maker CSL Ltd. also published results of a study that found between 75 and 96 percent of vaccinated people should be protected with one dose, although scientists initially thought it would take two.

Despite all the headlines about swine flu, which has become the main influenza strain circulating in the world, doctors do expect some garden-variety flu to hit this fall too — the kind that kills 36,000 Americans and hospitalizes 200,000 every year. The winter flu vaccine is widely available now, and U.S. health authorities are urging people to get it out of the way now before swine flu shots start arriving in mid-October.

Waiting to get the first inoculation out of the way “is not in anybody's best interest,” added Dr. Nancy Nielsen, past president of the American Medical Association. She said busy doctors need to have completed regular vaccinations by the time they have to deal with H1N1 shots.

And there's no way to predict how much of either flu strain will circulate. “This year, we are in uncharted territory,” warned Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention.

He said some parts of the Southeast in the past few weeks have experienced as much flu as is usually seen in the middle of winter. So far, it's all the H1N1 variety, with schools and colleges experiencing outbreaks almost as soon as classes began. Indeed, a typical school student who catches swine flu will spread it to two to three classmates, says a new estimate published Thursday in Science.

The study found the same side effects people experience with regular flu vaccine, which is no surprise since this shot is merely a recipe change from the annual standby. About 45 percent of recipients had mild reactions such as a headache, sore arm or redness at the shot site.

One dose means tight supplies of H1N1 vaccine won't be stretched so badly after all. The U.S. has ordered 195 million doses, based on the hope that 15 micrograms was indeed the right dose. Had it taken twice that dose, or two shots apiece, half as many people could have received the vaccine.