JIM LEHRER: Next tonight, new hope for an AIDS vaccine more than two decades after the disease was first diagnosed. Tom Clarke of Independent Television News has our report.
TOM CLARKE: This is Thanad Yomha, a public health volunteer in Thailand's Rayong province. This area has some of highest rates of HIV in the country, making it an ideal place to study the virus.
Now, after six years of work here, the largest ever trial of AIDS vaccines is complete, it suggests what some scientists had begun to think impossible: the human immune system can be armed against HIV.
DR. SUPACHAI RERKS-NGARM, director, Thailand AIDS Vaccine Trial: This is a scientific breakthrough. This is the first time that we can prove a vaccine can be developed to prevent infection among the vaccine, which is a first stepping stone for further vaccine development.
TOM CLARKE: In the last 26 years, so many AIDS vaccine trials have failed, many scientists argued it was unethical to attempt this one. But the Thai ministry of health and the U.S. Army, which funded this research, pushed ahead. Smaller studies suggested two vaccines, which had previously failed, might just work in combination.
HIV infects immune cells. That's why it's been such a challenge for vaccine researchers. The virus hides within the very immune system vaccines are designed to stimulate. This trial used two existing vaccines made from a different component of HIV. The first, called ALVAC, contains three HIV genes wrapped up in a disabled virus that's harmless to humans. The second, called AIDSVAX, is made from a protein called gp120 that covers the surface of the virus.
Over 16,000 people were involved in the Thailand trial. Of the 8,197 given the vaccine combo, 51 became infected with HIV; of the same number who got dummy shots, 74 became infected. A modest result, but statistically, that equates to a 31 percent reduced risk of infection.
There's still a lot to explain, like why two-thirds of the volunteers who went on to contract HIV despite having received jabs got as sick as those who got no vaccine. If vaccines don't prevent infection, they should make it milder. They also need to see if the vaccines work against other strains of HIV and in people with different genetic backgrounds.
DR. ROBIN SHATTOCK, St. George's Hospital Medical School, England: It's the first time in the history of HIV vaccine research that we've had a result that gives us a positive effect. But the effect is relatively modest, so it's not as if the job is done, but it gives us something really to work on and a way forward to hopefully developing a truly preventative vaccine.
TOM CLARKE: Every day, 7,000 people become infected with HIV. Most cases are still in sub-Saharan Africa, though the disease is expanding in Eastern Europe. While hopes for a vaccine may have been re-kindled today, it's a long way off at best. Even proponents warn the emphasis should remain on other forms of protection.
DR. CATHERINE HANKINS, chief scientific adviser, UNAIDS: Correct and consistent use of male and female condoms to changes in behavior, sexual behaviors, injecting behaviors, male circumcision. We have a list of things that we know are effective, and we've got to scale those up now while we're working on a vaccine.
TOM CLARKE: Thanad Yomha knows he might not see a vaccine in his lifetime.
THANAD YOMHA, Thai public health volunteer (through translator): I didn't expect anything. I did this for others. And as for me, considering how old I am, what we all do now is for the next generation.
TOM CLARKE: At least now his successors know an HIV vaccine isn't too much to hope for.