WASHINGTON (AP) -- Influenza is circulating unusually early this year with cases in all 50 states -- nearly all the swine flu variety, government health officials said Friday.

The highest concentration of flu cases is in the Southeast and a few other states, Dr. Anne Schuchat of the Centers for Disease Control and Prevention said at a briefing.

The good news is that testing of vaccines for swine flu show that they work with a single dose and take effect rapidly.

Supplies of swine flu vaccine are expected to be available in mid-October, but the seasonal flu vaccine is available now and officials have encouraged people to get it.

The H1N1 swine flu broke out in the spring and never went away, Schuchat said. It struck in many summer camps, spread into the Southern Hemisphere and now is widening its range. Currently 98 percent of the flu viruses circulating are swine flu.

Cases are mainly occurring in children and young adults, Schuchat said.

The finding that the swine flu vaccine works in a single dose in healthy adults "shortens the window of worry," Health and Human Services Secretary Kathleen Sebelius said. "There's no better protection against the flu than vaccine."

There had been concerns that it would take two doses to build up immunity, delaying the protection.

While the single dose works in adults, testing is still under way to determine the effectiveness of the vaccine in children and pregnant women, said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Disease.

He said the tested vaccines were made by Sanofi Pasteur and CSL Ltd. and both produced "robust" immune responses.

In people aged 18 to 64, 96 percent had a strong response to the Sanofi version and the response was 80 percent for CSL. Fauci played down the difference, noting the tests were done after only eight to 10 days and immune response could be the same in both groups as it increases after that point.

In addition, there were no significant side effects, Fauci said.

People over 65 did not respond as strongly, but still got enough of an immune reaction that they should seek out the shots when their turn comes, officials said.

First on the list for the swine flu shots, however, are children and young adults, pregnant women and others with health problems, since the H1N1 flu seems to strike them more often.

Older people are more at risk from the regular seasonal flu and -- along with other people -- should get those shots now, Sebelius said.

She noted she got her own seasonal flu shot Friday at a school in nearby Alexandria, Va.

Why bother with the seasonal shot, since nearly all the current flu cases are swine flu?

"The fact that the (seasonal) virus is not circulating now is absolutely no reason not to get vaccinated," Fauci said. "You would hope that you would get vaccinated before the seasonal flu is circulating so you will have an immune response."

Fauci said it still appears the bulk of the swine flu vaccine will be available in mid-October, though there is a possibility some may be available sooner, "we hope."

"The disease is increasing already and it is still a bit of a race to get the vaccine out there ahead of the disease," Schuchat said.

Even with the swine flu spreading now there will still be plenty of need for the vaccine, the officials stressed.

One dose means tight supplies of H1N1 vaccine won't be stretched so thin 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 dosage, or two shots apiece, half as many people could have received the vaccine.

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