"There's only been one other trial in humans ever before," says Dr. Jeffrey Laurence of Laboratory for AIDS Research.
Scientists at pharmaceutical giant Merck & Co. have developed a vaccine they say has shown promising results in monkeys. However, they remain cautious, saying most projects that reach this stage fail in clinical development.
Many larger pharmaceutical companies have been unwilling to invest in research on an AIDS vaccine partly because of the daunting scientific challenges, not to mention the enormous costs.
In fact, research almost stopped in the mid-1990's after two promising vaccines failed critical lab tests. But companies are now rebounding and trying again.
"The Merck decision to move into trials shows that they see that there's a potential profit and potential market for their investment," says Sam Avrett of the AIDS Vaccine Advocacy Coalition.
With 16,000 people becoming infected every day with HIV -- the virus that causes AIDS -- the market is certainly there. AIDS patients now pay upwards of $10,000 a year to treat the disease. A vaccine would prevent it altogether.
"We need an AIDS vaccine. There has never been a viral epidemic on the planet that has ever been solved with drugs. It's always been a vaccine," says Laurence.
Companies other than Merck have also re-entered the vaccine field. But even if some of their preliminary tests are successful, researchers warn there are still plenty of hurdles that remain before a commercial vaccine could develop.