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Last month, scientists reported that a clinical trial for an HIV vaccine showed the first-ever success in preventing transmission of the virus. However, a number of HIV researchers believe that the enthusiasm for last month’s vaccine results should be dampened in light of a more comprehensive review of the data.

According to scientists interviewed by Nature, the authors of the paper that reported the vaccine trial used an analytical method that disregarded initial trial candidates that either dropped out of the trial or failed to strictly follow the vaccine regimen. When those initial candidates are factored into the analysis, the number of people protected by the vaccine drops below statistical significance. The critics maintain that including those people in the analysis of the data is important both for understanding how the vaccine might work under real world conditions, and to maintain a random sampling of people as trial participants.

“The results of this trial should be treated with caution and some scepticism,” Tim Peto, a researcher in tropical diseases and clinical medicine at the University of Oxford, told Nature. “Taken together with the disappointing results of previous vaccine studies, it is likely that the results could have been due to chance alone.”

While the skeptics quoted by Nature do note that the deeper analysis of data does not necessarily invalidate the success of the vaccine, it does make it more difficult to determine whether or not the vaccine was successful. Additionally, they also note that in a field like this, any progress, even diminished progress, is good news.

[via Nature]