A study published in the journal Nature late last month did something no HIV vaccine had done before: triggered a level of protection HIV fighting antibodies In monkeys with completely normal and unmodified immune systems.
That’s a narrow result — only one animal in the test reached that protective threshold, and less than half showed any antibody response. But the researchers behind the work say its significance lies less in its scale than in how it was achieved, after more than a decade of research and a long string of previously unsuccessful HIV vaccines.
The vaccine was developed by a consortium that included Scripps Research, La Jolla Institute for Immunology (LJI) and the Neutralizing Antibody Center of the International AIDS Vaccine Initiative, as well as Fred Hutch, Emory’s Primate and Vaccine Center, the Ragon Institute of Massachusetts General Hospital, MIT and Harvard, the Howard Hughes Medical Institute, and the University of California, San Diego. It is the product of a 14-year collaboration between LJI and Scripps Research under the Scripps Consortium for HIV/AIDS vaccine development.
“This feels like a huge breakthrough. We built a successful vaccine from the ground up, which required a deep understanding of the immune system,” LJI Chief Scientific Officer Shane Crotty, who co-led the research with William Schiff of Scripps Research, said in a statement.
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Why is it so difficult to make an HIV vaccine?
HIV resists vaccination for three reasons, all of which have to do with how the virus behaves once it’s inside the body. It hides itself in a layer of sugar molecules, called glycans, that also cover human cells, allowing it to move largely undetected by the immune system.
It also mutates at an extraordinary rate. “The worldwide diversity of HIV mutations is extraordinary. Even the diversity within a single person living with HIV is dramatic,” said Patrick Madden, co-first author of the study with LJI instructor and Scripps Research investigator John Steichen.
And once it infects a cell it changes its shape and often outwits the immune response along the way.
Those three hurdles have defeated nearly every HIV vaccine in reaching large-scale human testing. RV144, which was tested in Thailand and results were reported in 2009, is the only vaccine to reduce the risk of infection in human trials – 31% efficacy over three and a half years, a result too weak to bring to market.
A vaccine developed by Merck, its trial known as STEP, was halted in 2007 after monitors found evidence that it was increasing rather than reducing the risk of infection in some vaccinated men.
A redesigned version of the Thai diet tested in South Africa a decade later as HVTN 702 showed no protective effects.
Two more vaccines, produced from mixed fragments of multiple HIV strains and tested as Imbocodo and Mosaico, were both halted in 2021 and 2023, respectively, due to lack of efficacy.
None of those vaccines used the strategy behind this new result. Germline targeting has never been tested on that scale, and the researchers are careful to note that it hasn’t failed with others — it just hasn’t been tried at that scale yet.
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Three-Phase Regimen: Prime, Boost, Polish
Most vaccines work by presenting the immune system with a piece of the virus and hoping that, among the body’s countless responses, a useful response will emerge. Germline targeting reverses that logic. Antibodies are produced by immune cells called B cells, which begin in an early “naive” state before maturing into effective antibody producers.
Instead of presenting an antigen and hoping, pathogen targeting starts with the antibody the vaccine wants to produce in the body, finds rare cells that are already capable of making it, and trains them directly there. Think of it as recruiting someone who can already do the job, rather than advertising a vacancy and hoping the right applicant comes along.
That recruitment takes place in three distinct stages, each playing a different role.
The priming shot is designed to find and activate an exceptionally rare class of naïve B cell – one that already possesses the specific genetic starting material needed to produce broadly neutralizing antibodies. Most naive B cells do not have this raw material. Priming is designed to recognize and activate the immunogen.
In this study, researchers engineered a priming immunogen that targeted a specific site on the outer “enveloping” protein of HIV – known as BG18 – and then tested the regimen in rhesus macaques.
Subsequent boosters do not repeat the priming shot. Each is engineered to reproduce more features of the real, original virus than the one before, exposing newly activated B cells to increasingly complex, real-life versions of HIV’s structure. In fact, it allows the kind of viral diversity that has doomed most vaccines built around a single, immutable antigen.
The final “polishing” phase is where the regimen tries to close the gap between an antibody that only recognizes HIV and one that can neutralize it, refining the antibody’s structure until it can bind and disable a wide range of true HIV strains, not just the version of the virus that was shown during priming and boosting.
“This series of vaccinations will guide, or ‘drive,’ the B cell from its naive state to a broadly neutralizing state,” Madden said, according to the LJI statement.
The researchers described the overall process, more informally, as a kind of training program: A rare cell is found, then it is run through a structured sequence to make it capable of doing something only a small number of people, in a small number of people, can normally do on their own.
Researchers say this is a deliberate design that makes even a modest result seem significant, compared to an area where most vaccines have worked by trial and error, and have mostly failed.
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reading the results correctly
The study presented two different data measuring two different things. In more than half of the animals, the correct B cells matured along the intended path, with neutralization potency reaching 67% of the potency of a reference antibody – evidence that the vaccine was doing its job at the cellular level.
On a narrower and tougher measure – the actual neutralizing antibodies circulating in the blood – the response rate was 44%. It was only within that small group that the best-performing animals reached antibody levels that the study authors described as expected to provide protection.
Even this conclusion comes with an important caveat: The vaccinated animals were not later exposed to HIV, so the study doesn’t show that the antibodies actually prevent infection — in only one animal, they reached the protection threshold. “It was incredible to get those results, but of course we would like to see a response in 100% of the animals,” Madden said.
the way forward
Part of this vaccine’s targets have already been tested in humans.
A US trial called HVTN 144 evaluated whether the priming immunogen alone could activate the correct precursor B cells for the BG18 antibody class in people – and it could. A follow-up trial, IAVI G004, began in December 2025 and is now conducting next-phase testing using an mRNA delivery platform manufactured by Moderna, where Schiff, one of the study’s senior authors, is now based. The full three-step method used on monkeys has not yet been tested on humans. IAVI, Scripps Research and the HIV Vaccine Trials Network said they are pursuing plans to do so.
Researchers caution that even a clean result in a future trial would only show that the vaccine can produce the right antibodies in a person, not that those antibodies protect against HIV. This would require much larger efficacy trials, to be run on the scale of RV144 or the vaccines that have failed since, one step germline targeting has not come close to achieving.
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What this could mean for medical advances
Nature published a related finding on a different HIV antibody target from a different research group in April 2026, and on the same day the paper came out, a second study on the same antibody target from another group was published. Three papers, from at least two other laboratories, converged on the broader strategy within a two-month period.
Researchers say the pattern shows that many groups are approaching similar milestones, rather than an isolated breakthrough.
IAVI, a co-developer of the study, said the effects could extend beyond HIV. If vaccine design can reliably advance antibody development toward a predetermined target, the same approach could eventually inform vaccine efforts against other viruses that mutate rapidly and evade the immune system. This possibility, like the vaccine, remains unproven.
