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It worked only on two strains of a highly mutable virus, yielded modest results and is nowhere near commercial licensing. But the first successful trial of an HIV vaccine has given a huge boost to scientists seeking to inoculate humankind against the virus linked to AIDS.

The results were based on a sliver of difference between two large test groups in Thailand, but were promising enough, considering that researchers have tried in vain for 26 years to come up with an effective vaccine.

“We're on first base. This is not a home run,” Alan Bernstein, executive director of the New-York-based Global HIV Vaccine Enterprise, said in an interview.

However, he said, the vaccine will be a springboard for future research. “When we get there, eventually it'll save hundreds of millions of lives. This is an important day for the planet,” Dr. Bernstein said.

“It's the first major hope in the field in a long time. … It gives us something to build on” said Mark Wainberg, director of the McGill AIDS Centre.

“The field of HIV vaccine was under a very dark cloud over the last 18 months as a result of the repeat failures of various candidate vaccines,” said Julio Montaner, president of the International AIDS Society and director of the B.C. Centre for Excellence in HIV/AIDS.

Dr. Montaner noted that the reasons for the latest vaccine's success aren't fully understood yet. “There might be light at the end of the tunnel. How bright, it's not clear. How long is the tunnel, it's not clear either.”

The study was sponsored by the U.S. Army and conducted by the Thai Ministry of Public Health.

Out of about 16,400 test subjects, half of whom were getting a placebo and the others the vaccine, 23 fewer people got infected among the inoculated group.

That small difference in infected participants – 51 among 8,197 vaccinated volunteers and 74 among 8,198 people on placebo – explains the vaccine's 31-per-cent efficiency rate.

The vaccine was based on two HIV strains found in Thailand, Subtype E and B. Subtype B is also common in Europe and the Americas.

However, it is Subtype C that predominates in Africa and is responsible for most of the world's infections, Dr. Wainberg said.

“It remains to be seen if the two specific vaccine components in this particular regimen would be applicable to other parts of the world,” the World Health Organization and the UN Program on HIV/AIDs said in a joint statement.

“Once an HIV vaccine does become available, it will need to be universally accessible by all persons at risk.”

Another downside is that vaccinated volunteers who still became infected ended up with the same amount of virus as those who hadn't been inoculated. Normally, vaccines should lower a person's viral load.

Dr. Bernstein said this might be due to the still little-understood interaction between the vaccine's two components.

The test combined two previous failed vaccines. ALVAC-HIV uses a harmless bird pox virus to deliver portions of the HIV envelope to induce the formation of antibodies. AIDSVAX replicates the gp 120 spikes, the protein appendages found on the outer shell of HIV, to stimulate the immune system into attacking the viral invaders.

The six-year, $105-million trial was conducted in the coastal provinces of Rayong and Chon Buri in southeastern Thailand. Volunteers received six injections over six months, four initial doses of ALVAC, with two booster shots of AIDSVAX.

The successful trial has already triggered public reactions suggesting some might be lulled into a false sense of security. “Now that they have a HIV vaccine do ya'll think people will feel safe having sex with no condoms?” read one Twitter posting Thursday.

Even if a future vaccine is effective enough to pass regulatory approval, it would have to be used along with other tools such as public education and increased HIV screening, Dr. Montaner said.

Dr. Bernstein said the Thailand results have opened the door to many research possibilities.

“Governments need to step up to the plate and recognize that they have now a tremendous opportunity to help end this epidemic.”