An HIV vaccine does not yet exist, but it is one of the most actively pursued goals in infectious disease research. Scientists have been working on it for over 40 years, and the virus’s extraordinary ability to mutate and evade the immune system has made it one of the hardest vaccines ever attempted. Two types are under development: preventive vaccines, designed to stop HIV infection before it happens, and therapeutic vaccines, intended to strengthen the immune response in people already living with HIV.
Preventive vs. Therapeutic Vaccines
A preventive HIV vaccine would work like most vaccines you’re familiar with. It would be given to people who don’t have HIV, training the immune system to recognize and fight the virus if exposure occurs. The goal is straightforward: stop infection before it takes hold.
A therapeutic HIV vaccine takes a different approach. It’s designed for people who already have HIV, with the aim of boosting their natural immune response against the virus. This could potentially reduce dependence on daily antiviral medications or help the body control the virus more effectively on its own. Neither type is available yet, but both are in active development.
Why HIV Is So Hard to Vaccinate Against
HIV mutates faster than almost any virus scientists have tried to build a vaccine for. The outer surface proteins of the virus, the parts your immune system would normally learn to recognize, are the most variable of all. Between different strains within the same family, these surface proteins can differ by about 15% in their building blocks. Between different families of HIV circulating around the world, that gap widens to more than 35%. This means a vaccine built around one strain is unlikely to protect against the full range of HIV variants a person might encounter.
The virus also evolves in real time inside a single person’s body, continuously reshaping its surface to dodge immune defenses. This is fundamentally different from viruses like measles, which look essentially the same to the immune system decade after decade. HIV’s outer coat has evolved to tolerate enormous variation without losing function, giving it a built-in escape mechanism against immune responses. The combination of global diversity and rapid in-body mutation means that a vaccine targeting only one version of the virus won’t provide lasting, broad protection.
The Broadly Neutralizing Antibody Strategy
The most promising current approach centers on a rare type of immune defense called broadly neutralizing antibodies, or bnAbs. Unlike standard antibodies, which typically recognize only one specific variant of a virus, bnAbs target parts of HIV that stay the same even as the virus mutates. Scientists view them as the immune system’s best realistic weapon against a virus this variable.
The problem is that the body almost never produces bnAbs on its own, even during natural HIV infection. So researchers are designing vaccines that guide the immune system through a multi-step process to manufacture them. The first dose, called a priming vaccine, activates a small population of rare immune cells (a type of white blood cell called B cells) that have the potential to eventually produce bnAbs. This technique is called germline targeting. Later booster shots then shepherd those activated cells through a maturation process, nudging them closer to producing antibodies broad enough to block many HIV variants at once.
Early clinical trials of this strategy, led by Scripps Research and partners, have shown that the concept works. The priming step successfully activated the right B cells, and a two-dose sequence moved the immune response further along the path toward bnAb production. These trials weren’t designed to generate full bnAbs yet, but they demonstrated that the step-by-step approach is viable in humans.
A History of Setbacks
Only one HIV vaccine trial has ever shown a hint of protection. The RV144 trial in Thailand, which enrolled over 16,000 people and reported results in 2009, found a modest vaccine efficacy of 31.2% in the primary analysis. That number was just barely statistically significant, and it faded over time. Still, it was the first real evidence that a vaccine could offer any protection at all, and it reshaped how researchers think about immune responses to HIV.
Since then, several large trials have ended in failure. The HVTN 702 trial attempted to build on the Thai results in South Africa but showed no efficacy. The Imbokodo trial, testing a different vaccine strategy in women across sub-Saharan Africa, was stopped after interim analysis showed no meaningful difference between the vaccine and placebo groups. The Mosaico trial, which tested a similar approach in men and transgender individuals across the Americas and Europe, was also discontinued for the same reason. In each case the vaccines were well tolerated and safe, but they simply did not reduce HIV acquisition. The Mosaico failure was attributed in part to the vaccine’s inability to induce the broadly neutralizing antibodies needed to handle HIV’s diversity.
What’s in the Pipeline Now
The most active area of current development uses mRNA technology, the same platform behind COVID-19 vaccines, to deliver HIV vaccine candidates. Two Phase 1 trials are testing mRNA-based approaches developed in collaboration with Moderna.
One trial, sponsored by the National Institute of Allergy and Infectious Diseases, is testing three mRNA vaccines designed to trigger a different class of bnAb-precursor immune responses. Healthy, HIV-negative adults receive the vaccine and then booster doses at two and six months. The other trial, run by the International AIDS Vaccine Initiative (IAVI) with Scripps Research, uses the germline-targeting strategy described above: a first mRNA vaccine activates the right B cells, followed by a second, different mRNA vaccine that pushes those cells further toward producing broadly neutralizing antibodies. Both trials are enrolling adults aged 18 to 55 at sites across the United States, and IAVI has expanded testing to sites in South Africa and Kenya as well.
These are early-stage safety and immune-response trials, not efficacy studies. Even under optimistic timelines, a proven HIV vaccine is likely years away. But the mRNA platform offers real advantages: vaccines can be manufactured quickly, updated as needed, and designed to present HIV’s surface proteins in precise configurations that guide the immune system more effectively than older vaccine technologies could.
How Current Prevention Compares
In the absence of a vaccine, the primary biomedical tool for preventing HIV is PrEP, or pre-exposure prophylaxis. Daily oral PrEP reduces the risk of sexually acquiring HIV by close to 99% when taken consistently. The challenge is adherence. In one study of more than 13,000 people, 52% discontinued daily oral PrEP, and only 60% of those who stopped eventually restarted.
A newer option, long-acting injectable PrEP given every eight weeks, has shown superior results. In a head-to-head trial, the injectable form reduced HIV seroconversion to 0.57% compared to 1.7% with daily pills, a difference significant enough that the trial was stopped early. The injection schedule makes adherence easier to track and maintain, since every dose is administered in a clinic.
PrEP is highly effective but requires ongoing access to healthcare and a commitment to either daily pills or regular injections for as long as a person remains at risk. A vaccine, even a moderately effective one, would change the equation dramatically. Mathematical modeling has estimated that vaccinating 80% of the adult population with a vaccine that’s only 40% effective could reduce cumulative new HIV infections by 73% over a 30-year period. Even accounting for behavioral changes like reduced condom use, the reduction would still be substantial at around 42%. A vaccine wouldn’t need to be perfect to save millions of lives.