Promising HIV Vaccine Candidate Shows Strong Results in Trial
A small trial conducted across ten research sites in the U.S. revealed an HIV vaccine candidate achieving a breakthrough that scientists have sought for nearly forty years: it consistently generated powerful, virus-blocking antibodies in the majority of participants.
The study, published last week in Science Translational Medicine, focused on three experimental vaccines utilizing mRNA technology similar to that of COVID-19 vaccines, but with a key modification. Instead of encoding HIV’s outer protein in a free-floating form, two of the vaccines prompted cells to create a version anchored to membranes, closely resembling how the protein appears on the actual virus.
This slight adjustment made a significant difference. Following three doses, 80% of participants who received the membrane-bound vaccine developed “tier 2” neutralizing antibodies, which are indicative of robust protection against the virus. In contrast, only 4% of those who received the standard soluble form achieved this.
“The difference is pretty striking,” said Sharon Lewin, head of the Peter Doherty Institute for Infection and Immunity, who wasn’t part of the research. “These are the first studies, so they’re very, very important.”
Challenges in Developing an HIV Vaccine
Since its identification in 1983, HIV has proven exceptionally elusive. The virus is cloaked in a thick layer of sugars, concealing vital targets from antibodies, and it mutates so rapidly that even an immune system capable of tackling one strain is often ineffective against another.
For years, most vaccine candidates have focused on the “envelope trimer,” a three-part protein that HIV employs to attach to immune cells. However, much of the base of this trimer is shielded by the viral membrane, and traditional soluble protein vaccines expose it. This inadvertently leads the body to produce antibodies that bind there but are ineffective at preventing actual infections.
The novel strategy developed by William Schief and colleagues at Scripps Research and Moderna effectively anchors the protein to cell membranes. Consequently, the immune system learns to identify more vulnerable, real-world aspects of the virus, enhancing its ability to eliminate it.
Over 100 HIV vaccine trials have been conducted over the years, but this one stands out as particularly hopeful.
Prior animal tests suggested this new approach could be more effective, and this human trial confirmed those hints.
Insights from the Trial
The study included 108 healthy adults aged 18 to 55, who were randomly assigned to receive one of three vaccines: the soluble trimer, a membrane-bound trimer, or a modified membrane-bound version designed to block CD4 receptor binding. Each participant received either a low or high dose three times over a six-month period.
Blood tests showed that the membrane-bound vaccines elicited a stronger immune response than traditional methods. They appeared to guide the immune system to concentrate less on non-essential parts of the virus and more on its weaknesses—regions known as V1/V3 and C3/V5—which are likely to obstruct HIV.
Moreover, these vaccines promoted a reserve of memory B cells, which act as long-lasting defenders, and activated specialized T cells, which coordinate the body’s entire immune response. It was a full-scale mobilization of the immune system.
However, there were some limitations. The antibodies produced were “autologous,” meaning they were tailored to the specific virus strain used in the vaccine, but had limited efficacy against the diverse strains globally. The ideal outcome would be to produce broadly neutralizing antibodies (bnAbs) that can target conserved sites across various strains.
Side effects were notable, but not particularly alarming.
Seven participants, approximately 6.5%, experienced hives—an allergic reaction—after vaccination. In five cases, the hives became chronic, lasting more than six weeks, and some symptoms persisted for years. This reaction was noted across all vaccine types and dosages, which has not been reported with mRNA vaccines for other viruses.
“It is a scientific mystery at the moment,” Schief remarked.
Researchers suspect that the interaction of HIV proteins with the mRNA delivery method triggers this response, but they have yet to determine the precise cause. Future trials may explore lower doses to minimize this risk.
Prospects for an HIV Vaccine
Despite many uncertainties, the study marks a significant milestone. Only two other mRNA HIV vaccine trials have progressed to human testing, and until now, none have demonstrated such a high production rate of neutralizing antibodies. This is genuinely promising.
Experts believe that mRNA technology’s speed and adaptability could streamline the challenging process of training the immune system to combat HIV. In the future, scientists might develop a single mRNA shot that releases a complete sequence of priming and boosting proteins in a timed manner, potentially eliminating the need for numerous clinic visits.
Yet, challenges remain. More extensive studies are necessary to establish safety and efficacy, and researchers still need to devise ways to target multiple strains effectively.
But it’s a worthy task.
Even the most effective current HIV prevention methods, like the semi-annual injectable drug lenacapavir, necessitate ongoing use and access to medical care. A long-lasting, broadly protective vaccine could be revolutionary, particularly in areas with limited resources.
