Chinese pharmaceutical firm Sinovac Biotech is using 1,600 volunteers in human trials of its vaccine against the H1N1 flu strain.
They're being given one dose now and another three weeks later to work out the right amount to give.
[Yin Weidong, GM, Sinovac Biotech Ltd]:"Through this trial we can work out the right dosage. We want to use at little antigen as possible. Of course, safety comes first and low dosages of antigen must be based on safety. Then we can think about using as little of it as possible to produce as many doses as possible."
There have been no deaths in China from the new bug, commonly known as swine flu, but there are more than 2,290 recorded cases.
Chinese authorities are keen to stop it from spreading through the wider population.
Sinovac Biotech says it's already received a government order for five million doses of vaccine.
The firm says it's expecting to start mass production when clinical trials finish in September.
It's among dozens of pharmaceutical companies in the race to develop a mass-produced vaccine.
The swine flu bug has spread to some 160 countries and killed about 800 people since it broke out in March.
The World Health Organization says the virus must be closely watched in case it mutates into something more severe.