The first HIV vaccine to produce tangible positive results has raised excitement across the field of HIV research, but the results could be due to chance, say experts.

The trial — dubbed the world’s largest with over 16,000 participants — was organized by the Thai and US governments to test a combination of two previously discontinued HIV vaccines. Of the 8,197 HIV- men and women given the vaccine combination, 51 ultimately contracted HIV, while 74 of the 8,198 HIV- men and women given the placebo had contracted HIV by the time the study was concluded, making the vaccine recipients 31.2% less likely to contract the disease

“This result is tantalisingly encouraging. The numbers are small and the difference may have been due to chance, but this finding is the first positive news in the Aids vaccine field for a decade,” said Dr Richard Horton, editor of the famous Lancet medical journal, as quoted by the BBC.

No other HIV vaccine study has produced any positive results so far.

The study’s authors cautioned that more work needed to be done to suss out the reasons behind the vaccines’ apparent success, and to eliminate random chance as an explanation.