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Scientists said Thursday that a new AIDS vaccine, the first ever declared to protect a significant minority of humans against the disease, would be studied to answer two fundamental questions: Why it worked in some people but not in others, and why those infected despite vaccination received no benefit at all.

The vaccine — known as RV 144, a combination of two genetically engineered vaccines, neither of which had worked before in humans — was declared a qualified success after a six-year clinical trial on more than 16,000 volunteers in Thailand. Those who were vaccinated became infected at a rate nearly one-third lower than the others, the sponsors said Thursday morning.

“I don’t want to use a word like ‘breakthrough,’ but I don’t think there’s any doubt that this is a very important result,” said Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, which is one of the trial’s backers.

“For more than 20 years now, vaccine trials have essentially been failures,” Fauci said. “Now it’s like we were groping down an unlit path, and a door has been opened. We can start asking some very important questions.”

It will still, however, take years of work before a vaccine that could end the epidemic, which has killed about 25 million people, can even be contemplated.

“We often talk about whether a vaccine is even possible,” said Mitchell Warren, the executive director of the AIDS Vaccine Advocacy Coalition, or AVAC. “This is not the vaccine that ends the epidemic and says, ‘OK, let’s move on to something else.’ But it’s a fabulous new step that takes us in a new direction.”

In which direction is still unknown. No one — including the researchers from the U.S. Army, the National Institutes of Health, the Thai Ministry of Public Health and two vaccine companies that tested the vaccine — knows why the vaccine gave even its weak indicator of success.

Experts generally disdain vaccines that do not protect at least 70 percent to 80 percent of those getting them. And this vaccine did not lower the viral loads of people who were vaccinated but caught the virus anyway, which was baffling because even mismatched vaccines usually do that.

Simply repeating the trial to confirm the results would be pointless, experts agreed.

The trial, the largest AIDS vaccine trial in history, cost $105 million and followed 16,402 Thai volunteers.

The men and women ages 18 to 30 were recruited from two provinces southeast of the capital, Bangkok, from the general population rather than from high-risk groups like drug injectors or sex workers. Half received six doses of two different vaccines; half were given placebos.

For ethical reasons, all were offered condoms, taught how to avoid infection and promised lifelong antiretroviral treatment if they got AIDS. They were then regularly tested for three years; 74 of those who received placebos became infected, but only 51 of those who received the vaccines did.