“It’s a grand reminder that H.I.V. 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.
Over two years, 63 of 1,109 participants who received the placebo were infected with H.I.V., compared with 51 of 1,079 participants who received the vaccine — giving the vaccine an efficacy rate of 25 percent.
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 H.I.V. for at least an initial period of time.
“But in South Africa, the higher rates of H.I.V. incidence means you need something much more potent,” Dr. 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 percent protection risked giving women a “false sense of security,” Dr. Gray said.
But a parallel trial that uses a different iteration 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.