Researchers at the weeklong International AIDS Vaccine Conference in Cape Town, South Africa—which ended October 17—voiced their disappointment in failing to discover an AIDS vaccine and said they are now forced to rethink their approach in combating the disease, Agence France-Presse (AFP) reports.

Last year, two advanced clinical trials of a vaccine created by pharmaceutical company Merck were terminated once it appeared they actually heightened the risk of AIDS infection. That news stunned scientists.

“They still don't understand exactly what happened,” Mitchell Warren of the AIDS Vaccine Advocacy Coalition told AFP. “That finding forces people to realign and look at new ways and new approaches to how we are going to find an AIDS vaccine because it was so surprising.”

Alan Bernstein, director of the Global HIV Vaccine Enterprise, said that new research involves newly discovered defenses in the human body called the innate immune system, which works as an early warning system for invading diseases.

We now know the window of opportunity to stop HIV may be only hours—at most, days—after infection, he explained. “That's reason to think this early warning system might be critical to activate if we are going to design a vaccine,” Bernstein said.

The difficulty in creating a vaccine is that HIV can mutate at such a rapid rate. While the setbacks have caused some to argue that funding could be better spent on other treatment or prevention efforts, others insist that vaccines provide the most cost-effective health intervention.

According to the article, about 30 trials are under way around the world. Results from a study in Thailand—which began in 2003—are expected in 2009.