The government called Wednesday for several thousand volunteers to start rolling up their sleeves for the first swine flu shots, in a race to test whether a new vaccine really will protect against this novel virus before its expected rebound in the fall.

Eight medical centers around the country are enrolling for a series of studies directed by the National Institutes of Health, and the first shots should go into volunteers' arms by the second week of August.

First, doctors will test different doses of the swine flu vaccine in healthy adults, including the elderly — two shots, given 21 days apart. If there are no immediate safety concerns, such as allergic reactions, the same testing quickly will begin in babies and children, said Dr. Anthony Fauci, head of NIH's National Institute of Allergy and Infectious Diseases.

Will the results come in time to guide the government's debate on whether to roll out a mass vaccination campaign starting in mid-October, one expected to target mostly school-age children and young adults?

"It's going to be very, very close," Fauci told The Associated Press.

By early September, scientists should have the first clue — how much immune protection that initial dose triggers. How much protection that second dose adds won't be known for yet another month.

Included in the government studies are vaccines made by Sanofi-Pasteur and CSL Ltd.

The study sites:

— University of Maryland School of Medicine, Baltimore

— University of Iowa, Iowa City, Iowa

— St. Louis University, St. Louis, Mo

— Baylor College of Medicine, Houston

— Children's Hospital Medical Center, Cincinnati

— Emory University, Atlanta

— Group Health Cooperative, Seattle

— Vanderbilt University, Nashville

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On the Net:

To search for "H1N1 influenza vaccine" trials: http://www.clinicaltrials.gov