In another setback in the long quest to prevent H.I.V. infection, a trial in South Africa has been shut down because an experimental vaccine was not working, federal health officials announced on Monday.
The trial, which began in 2016, followed one in Thailand that ended in 2009. That vaccine offered only modest protection against infection. Experts argued over how much, but the vaccine was no more than 30 percent protective.
Nonetheless, it was the only vaccine that had appeared to work at all.
“We hoped this vaccine candidate would work — regrettably, it does not,” said Dr. Anthony S. Fauci, director of the National Institute for Allergy and Infectious Diseases, which conducted the trial.
“Research continues on other approaches to a safe and effective H.I.V. vaccine, which I still believe can be achieved,” he added.
A vaccine against H.I.V., the virus that causes AIDS, is sorely needed. Even now, nearly 40 years after the start of the epidemic, 1.7 million people are newly infected each year — most of them in Africa, especially southern Africa, according to UNAIDS, the United Nations’ AIDS-fighting agency.
The trial — known as HVTN 702 but nicknamed Uhambo, which means “journey” in Zulu — included 5,407 young adult men and women in South Africa.
Last month, a safety-monitoring panel looked at early results and found that there were 123 infections among participants who got a placebo injection and 129 among those who got the vaccine.
That clearly indicated that the vaccine was not protective, but did not mean it was making participants more vulnerable to H.I.V., scientists said. A difference of just six infections in so large a pool of participants could have been due to chance.
The Uhambo vaccine had to be significantly changed from the one tested in Thailand because South Africa has a different dominant strain of H.I.V.
The vaccine used canarypox, a bird virus that can infect human cells but cannot multiply in them, to deliver into the body a protein found on the outer envelope of H.I.V. The immune system learns to recognize the protein and to make protective antibodies to it.
“This is a big disappointment, and it means that canarypox is clearly not a road to success,” said Mitchell J. Warren, executive director of AVAC, an H.I.V.-prevention advocacy group based in New York. “But it was a well-conducted trial — it enrolled thousands of participants, and they came back for their tests.”
“As they say in the pharmaceutical industry, ‘If you’re going to fail, fail fast,’” he added. “That way, you don’t waste financial and human resources.”
Two other H.I.V. vaccine trials, Nos. 705 and 706, known as Imbokodo and Mosaico, are still underway. Both use a common cold virus as the vector and different surface proteins.
Imbokodo is enrolling women in five southern African countries, and Mosaico is enrolling gay men in Europe and North America.
For ethical reasons, trial participants were also offered pre-exposure prophylaxis or PrEP — a daily pill that prevents H.I.V. infection — as well as condoms and advice about how to avoid becoming infected.
But 30 years of condom distribution and counseling have failed to curb the raging H.I.V. epidemic in southern Africa. Several PrEP trials have failed there, too, because people enroll in them but then do not take the pills. So a vaccine remains an important goal.
Scientists are also testing other methods of stopping H.I.V.: injections of cocktails of antibodies that can block it, injections of long-lasting H.I.V. drugs, and implants and vaginal rings that release small amounts of preventive drugs over time.