An advanced HIV vaccine trial in Africa has been shut down after data showed the shots offered only limited protection against the virus, researchers said Tuesday.

The vaccine, made by Johnson & Johnson, is one in a long line found to offer little defense against HIV, one of medicine’s most intractable adversaries. One candidate vaccine even increased the risk of infection.

Another trial was halted last year in South Africa after a different experimental vaccine failed to offer sufficient protection. Some 1.5 million people were infected with HIV worldwide in 2020, and 38 million are living with the infection.

Scientists were dismayed by the most recent failure.

“I should be used to it by now, but you’re never used to it — you still put your heart and soul into it,” said Dr. Glenda Gray, the principal investigator of the trial and chair of the South African Medical Research Council.

Gray has been working to develop an HIV vaccine for more than 15 years.

Entirely new approaches may be needed. Moderna said earlier in August that it would test a vaccine based on the mRNA platform used to devise the company’s coronavirus vaccine.

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The trial, called Imbokodo, tested an experimental vaccine in 2,600 young women deemed at high risk of HIV infection in five sub-Saharan African countries. Women and girls accounted for almost two-thirds of new HIV infections in the region last year.

The trial was funded by Johnson & Johnson, the Bill & Melinda Gates Foundation and the National Institutes of Health.

The vaccine relied on an adenovirus called Ad26, modified to carry fragments of four HIV subtypes into the body in hopes of provoking an immune response that might defend against infection.

Mitchell Warren, executive director of AVAC, an advocacy group that lobbies for AIDS prevention and treatment, said the cancellation of the trial was a “reality check” amid excitement about new vaccine technologies.

“It’s a grand reminder that HIV is a pathogen unlike any other in its complexity,” he said. “We know the platform worked, but what do we put in it? Because this virus is infecting the exact same immune system that we’re trying to boost with a vaccine.”

Participants in the Imbokodo trial, which began in 2017, were given two initial shots and two boosters over the course of a year. Researchers tracked the numbers of new infections in the placebo and vaccine groups from the seventh month (one month after the third vaccination) through the 24th month.

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Over two years, 63 of 1,109 participants who received the placebo were infected with HIV, compared with 51 of 1,079 participants who received the vaccine — giving the vaccine an efficacy rate of 25%.

Earlier studies, including one carried out in Thailand, had indicated that the kind of antibodies this vaccine provoked might be sufficient to offer good protection from HIV for at least an initial period of time.

“But in South Africa, the higher rates of HIV incidence means you need something much more potent,” Gray said. “The kind of immune responses that were induced were just not enough to stop the high attack rates we see in Africa.”

When the disappointing data showed a low efficacy rate, guidelines set up before the trial dictated it should be shut down. A vaccine that offered only 25% protection risked giving women a “false sense of security,” Gray said.

A parallel trial that uses a different version of this vaccine will continue, Johnson & Johnson said. It is being tested on men who have sex with men and transgender people, in eight countries including Poland, Brazil and the United States.

That study, called Mosaico, is testing the vaccine against different subtypes of HIV in different populations, and it could produce different efficacy results.

Gray said that the lesson from the failed trial lies in figuring out why it worked for the 25% of people who were protected and not for the others, and then trying to translate those clues into a recipe for a future vaccine.