The reason for this decision was that the vaccine did not prevent HIV infection as expected.

The announcement was made on Friday, 21 September by the co-sponsors of this clinical trial, Merck & Co. Inc., and the HIV Vaccine Trials Network (HVTN), which is funded by the National Institute of Allergy and Infectious Diseases (NIAID), part of the US National Institute of Health.

This announcement followed the review of the results of a similar randomised placebo controlled study that was carried out in North America, South America and the Caribbean using the same vaccine.

As per protocol, the Data Safety Monitoring Board (DSMB) met to review the early data from this study last week.

The DSMB reviewed the results of the first 45 people who have become naturally HIV infected whilst on the study.

Normally, DSMB reviewers are confidentially shown the unblinded data to ensure that there are no indications that participants are being harmed by the intervention. T

hey also are expected to review whether the expected aims of the study are being met.

The results of this review showed:

Firstly, that the vaccine did not prevent HIV infection.

In 741 volunteers who received at least one dose of the vaccine, 24 volunteers became infected with HIV and in 762 who received the placebo, 21 volunteers became infected.

Secondly, the results show that in those who received the vaccine and became infected the vaccine did not reduce the amount of HIV virus in their body, which was the second aim of the trial.

The DSMB panel stressed that the safety of the volunteers in the trial was never at risk as the vaccine’s safety was firmly established in earlier studies.

Those who have taken part in the study should therefore have no concerns.

As a result of the findings of this sister trial, the Phambili HVTN 503 study has been “put on hold”.

All study sites in South Africa will continue to follow up participants who have been on the trial while the study sponsors and scientists decide on the way forward.

While these results may be extremely disappointing, the experience from this trial will provide scientists with very critical information about the HIV virus and future vaccine development.

Prof Linda-Gail Bekker, principal investigator for Phambili at the Desmond Tutu HIV Foundation, said, “This is good science – sorting out what works and what doesn’t as quickly as possible whilst not endangering anyone’s health.”