The latest trial of a vaccine against HIV has been halted because interim results show it is not working, the National Institutes of Health in the United States has announced.

The end of the trial taking place in South Africa is a blow to the vaccine field and to Aids experts and advocates. As early as the mid-1980s, the US government was forecasting that Aids would be stopped by a vaccine. In 1997, the then-president Bill Clinton pledged money to an effort to find a vaccine within 10 years. But as the decades have passed, no effective vaccine has been discovered.

The failure of the HVTN 702 study will be especially disheartening because it was set up in South Africa to trial the only vaccine that has ever shown any degree of success – the RV144 clinical trial in Thailand led by the US Military HIV Research Program and the Thai ministry of health. That trial showed only a modest protective effect, but in the absence of anything else, it was enough to put a substantial effort into testing that further.

The HVTN study, also called Uhambo, meaning travel or a journey in Zulu, enrolled 5,407 HIV-negative volunteers at 14 sites across South Africa, beginning in 2016. Participants were sexually active men and women aged 18 to 35 years, who were randomly assigned to receive six injections over 18 months of either the investigational vaccine regimen or a placebo.

But over a period of at least 18 months, there were 129 HIV infections among those who got the vaccine and 123 HIV infections among those who were given the placebo. The data and safety monitoring board recommended the trial be stopped.

“An HIV vaccine is essential to end the global pandemic, and we hoped this vaccine candidate would work. Regrettably, it does not,” said the director of the National Institute of Allergy and Infectious Diseases, Anthony S. Fauci. “Research continues on other approaches to a safe and effective HIV vaccine, which I still believe can be achieved.”

“The people of South Africa have made history by answering this important scientific question. Sadly, we wish the answer was different,” said Glenda Gray, president of the South African Medical Research Council and the study’s chair of protocol. “We will continue to explore promising avenues for preventing HIV with other vaccines and tools, both in South Africa and around the world.”

The International Aids Society (IAS), representing health professionals and researchers working in HIV, said there was deep disappointment at the outcome of a trial, for which there had been great hopes.

“Whilst this is a significant setback for the field, we need to continue the quest for a preventive vaccine. The rates of HIV infection, which continue unabated in this region, should spur greater urgency, global attention and investment to the quest,” said Linda-Gail Bekker, chair of its vaccine enterprise advisory group.

Roger Tatoud, the IAS deputy director of HIV programmes and advocacy, said: “A well-conducted trial, even if efficacy is not observed, plays a critical role in informing HIV vaccine development.” And there are other vaccines using different approaches still in human trials, he pointed out.