If the vaccine appears safe, the same testing will begin on infants and children.
The studies are critical because the government is considering whether to offer a Swine Flu vaccine to millions this fall. Scientists need to learn just how much protection one dose of the vaccine offers. They're hoping to have a good idea of that by early September.
Experts say it ought to be safe because similar to shots given for traditional flu each year.
The H1N1 virus has continued to spread -- four North Carolinians have died from it. About 400 have gotten sick.
It's feared the Swine Flu will grow stronger and have a resurgence this fall.