The first HIV vaccine to be called a success has stood up to scrutiny after further analysis of the data was presented today in Paris, France.

However, the new analysis also confirms that the optimistic claims, first made in September and viewed sceptically at the time, are indeed very modest.

Last month’s announcement of success (PDF) was made by researchers from the US Military HIV Research Program (MHRP). They reported that their vaccine reduced the risk of infection by about 31 per cent in a trial in Thailand.

But it was not clear that the vaccine offered any protection because the result was based on very few cases: 51 of 8197 vaccinated individuals became infected with HIV compared with 74 of 8198 unvaccinated people, a difference of just 23.


Bonus data

Today, at the AIDS Vaccine 2009 meeting in Paris, MHRP researchers presented the analysis underlying the result that they announced a month ago, plus two additional analyses of the raw data (PDF).

These new analyses included people who had been excluded from the research results, such as those who did not take the six vaccine shots in the correct order. In neither was the trend statistically significant.

After hearing the new results, Seth Berkley, chief executive of the International AIDS Vaccine Initiative, said, “Certainly, there’s some kind of signal there,” but added, “It’s a modest effect.”

He also says that the new results are interesting because they give novel insights into how the vaccine works over time.

Early effect

The risks of infection in the vaccinated group were reduced by around 60 per cent within a year, but by 30 months after vaccination the protective effect was only 36 per cent. This resulted in a 31 per cent figure overall.

“It looked like there’s an early effect that wanes with time,” said Berkley. “It may be that the vaccine generates only weak antibodies against HIV, and these are only effective early on.”

Berkley says that further investigation of the mechanisms by which the vaccine worked would provide powerful new knowledge to guide selection of new, more potent vaccines.

Journal reference: New England Journal of Medicine, DOI: 10.1056/NEJMoa0908492