Researchers have started the first human trial for a new swine flu vaccine, recruiting volunteers worldwide to test whether the formula protects against the rampant virus.
About 240 volunteers in Australia have already received the shot this week.
Some medical experts fear it's too soon to give the vaccine, saying additional research is needed.
Wednesday, U.S. government officials put out a call for volunteers to participate in an upcoming swine flu study.
The National Institutes of Health will be administering the vaccines during the second week of August in centers across the country.
Researchers say the tests will provide vital information as the government moves to offer swine flu vaccines to millions of Americans beginning in mid-October.
The vaccine is difficult to manufacture and health authorities question whether it will be ready by October. Other countries are also making plans and looking to the U.S. for its studies.
"It's going to be very, very close," said Dr. Anthony Fauci, director of NIH's National Institute of Allergy and Infectious Diseases.
Dr. Karen Kotloff of the University of Maryland School of Medicine, who is helping to lead the NIH study, agreed that researchers must work quickly.
"We are racing to provide them as much information as we possibly can," she said.