Target group: The volunteers primarily belonged to the low- and moderate-risk groups that were at risk of being infected through the heterosexual route.

The results of the Phase III AIDS vaccine trial using a prime-boost (ALVAC and AIDSVAX) vaccine undertaken in Thailand, published in the New England Journal of Medicine (NEJM), show that the vaccine is able to produce 31.2 per cent protection in those who are not infected at the time of enrolment.

This is the first time ever that a vaccine has been able to provide statistically significant protection. The trial was conducted on nearly 16,500 volunteers.

Despite this good news, the results clearly show that the vaccine cannot be used as a public health control measure.

That is because the volunteers primarily belonged to the low- and moderate-risk groups that were at risk of being infected through the heterosexual route. Nearly 48 per cent of the volunteers belonged to the low-risk group and 28 per cent belonged to the moderate-risk group.

The reality

The authors claim that the prime-boost vaccine tested “may reduce the risk of HIV infection in a community-based population with largely heterosexual risk.” But the Editorial published in the same issue of the journal makes it very clear that it cannot be used for public health control. “The vaccine regimen studied is unlikely to be a public health control measure for HIV-1 infection,” notes the Editorial.

Additionally, the researchers found that the duration of immune response, and hence protection offered, may have decreased in the first year after vaccination.

This is despite the fact that the vaccine tested was a prime-boost combination and six injections (four to prime the immune response and two to boost it) were administered.

They also found that the vaccine efficacy (ability to protect against HIV infection) was greater in people who were at low risk for infection. An acceptable vaccine for public health control will be the one that provides more statistically significant level of protection to both the high-risk and the low-risk groups.

The Editorial notes that the trial has brought out the fact that the “requirements for protection against transmission in low-risk, heterosexual persons are considerably different or less stringent than in high-risk subjects.” That could be the silver lining for further research in AIDS vaccine.

Unexpected result

Though both the vaccines failed when used alone in trials conducted earlier, the combination has been found to provide nearly 31 per cent protection. That appeared strange to many people working with AIDS vaccine.

However, the researchers have not been able to find the answer to how the combination protected some of the individuals. They note that the data was not able to answer the “related question of whether it was a single vaccine or the combination of vaccines that induced a potentially protective immune response.”

A prime-boost combination is supposed to produce qualitative or quantitative protective immune response that is not otherwise seen when the vaccines are administered alone.

But the study data has not addressed this important issue. Despite the several shortcomings and limitations, the trial has provided the much needed hope for future work on AIDS vaccine.

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