The first shots should go into volunteers' arms by the second week of August. A network of medical centers around the country is enrolling for the series of studies directed by the National Institutes of Health.
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.
"The questions are clear. One is: is it safe? And if so, what is the right dosage and number of doses? We need to ask that question both in healthy young adults, we need to ask the question in elderly individuals, and we need to ask the question in children," Fauci told CBS News medical correspondent Dr. Jennifer Ashton.
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 the second dose adds will not be known for yet another month.
"We'll have a good idea by measuring the antibodies whether or not this vaccine is going to work," Dr. Kathryn Edwards from the Vanderbilt University Medical Center told Ashton.
As for safety, the new vaccine is expected to be similar to shots already given each year for seasonal flu. "It ought to be extremely safe," said Dr. William Schaffner, a vaccine specialist at Vanderbilt University, where a colleague will help test the vaccine.
But some question the government's track record. In 1976 the decision to vaccinate 43 million people against a similar strain of flu backfired. The outbreak never materialized and approximately 500 people suffered severe side affects, while 25 died, reports Ashton.
"We frankly don't know why there was that rare adverse event in 1976. And There's never a hundred percent guarantee that you're not going to see an adverse event now," Fauci said.
Included in the government studies are vaccines made by France-based Sanofi-Pasteur and CSL Ltd. Also Wednesday, CSL began a much smaller study of its vaccine in Australia, where the company is based.
Soon, vaccine from additional manufacturers - including the only non-shot version, a nasal spray - will begin separate, company-led studies in several thousand more volunteers.
But the NIH studies are specifically designed to test how the swine flu vaccine might be used by average Americans, with a set of tests to see how it reacts if given before, with or after the regular winter flu vaccine - which people are supposed to get as usual later this year.