NIH on HIV Vaccine Failure: 'Get Your
HIV-Negative, At-Risk Patients on PrEP Tomorrow'

Heather Boerner

September 02, 2021

Last year, Katherine Gill, MBChB, an HIV prevention researcher in Cape Town, South Africa, realized how jaded she'd become to vaccine research when the Pfizer COVID-19 vaccine came back as 95% effective. In her career conducting HIV clinical trials, she'd never seen anything like it. Even HIV prevention methods she had studied that had worked, like the dapivirine ring, only had an overall efficacy of 30%.

The COVID-19 success story started to soften her views toward another vaccine trial she was helping to conduct, a vaccine trial in HIV that used the same platform as Johnson & Johnson's successful COVID-19 vaccine.

"When the COVID vaccine was cracked so quickly and seemingly quite easily, I did start to think, 'Well, maybe…maybe this will work for HIV,' " she told Medscape Medical News.

That turned out to be false hope. This week, the National Institutes of Health (NIH) announced that the trial Gill was helping to conduct, HVTN 705, was stopping early because it hadn't generated enough of an immune response in participants to justify continuing. It was the second HIV vaccine to fail in the last year. It's also the latest in what has been a litany of disappointments in the attempt to boost the human immune system to fight HIV without the need for ongoing HIV treatment.

In HVTN 705, known as the Imbokodo study (imbokodo is a Zulu word that's part of a saying about women being strong as rocks), researchers used the platform made up of a common cold virus, adenovirus 26, to deliver a computer-generated mosaic of HIV antigens to participants' immune systems. That mosaic of antigens is meant to goose the immune system into recognizing HIV if it were exposed to it.

When HIV enters the body, it infiltrates immune cells and replicates within them. To the rest of the immune system, those cells still register as just typical T-cells. The rest of the immune system can't see that the virus is spreading through the very cells meant to protect the body from illness. That, plus the armor of sugary glycoproteins encasing the virus, has made HIV nearly impervious to vaccination.

Then, those so-called "prime" shots were followed by a second shot that targets glycoprotein 140, on the most common HIV subtype (or clade) in Africa, clade C. In the Imbokodo trial, a total of 2637 women from five sub-Saharan African countries received shots at baseline, 3 months, 6 months and 1 year. Then researchers followed the women from month 7 to two years after their third dose, testing their blood to see if their immune systems had generated the immune response the vaccine was meant to induce — and whether such immune response was associated with lower rates of HIV.

When researchers looked at the first 2 years of data, they learned that the vaccine was safe. And they found a total of 114 new cases of HIV — 51 among women who received the vaccine and 63 among those who received a placebo. That's a 25.2% efficacy rate — but it wasn't statistically significant.

The results are frustrating, said Carl Dieffenbach, PhD, director of the Division of AIDS at the National Institute of Allergy and Infectious Diseases (NIAID). NIAID is one of the funders of the study.

"This [trial] is a little more confounding, in that there is this low level of statistically insignificant difference between vaccine and placebo that starts somewhere around month 9 and then just kind of indolently is maintained over the next 15 months," he told Medscape Medical News. "That's kind of frustrating. Does it mean there's a signal or is this just chance? Because that's what statistics tell us, not to believe your last data point."

What this means for the future direction of vaccine research is unclear. A sister study to Imbokodo, called Mosaico, finished enrolling participants this week. Mosaico uses the same adenovirus 26 platform, but it's loaded with different antigens and targets a different glycoprotein for a different HIV subtype. If that trial shows success, it could mean that the platform is right, but the targets in the Imbokodo vaccine were wrong.

Dieffenbach said that before NIAID decides what to do with Mosaico they've asked researchers to analyze the data on the people who did respond, to see if those people have some specific variant of HIV or some other biomarker that could be used to form the next iteration of an HIV vaccine candidate. Only after that will they make a decision about Mosaico.

But he added that it does make him wonder if vaccine approaches that rely on non-neutralizing antibodies like this one have a ceiling of effectiveness that's just too low to alter the course of the epidemic.

"I think we've discovered that there's not a floor to [these non-neutralizing approaches], but there probably is a ceiling," he said. "I don't know if we're going to get better than" a 25% to 29% efficacy rate with those approaches.

The Imbokodo findings reminded Mitchell Warren, executive director of the global HIV prevention nonprofit, AVAC, of the data released in January from the Antibody Mediated Prevention (AMP) trial. That trial pitted the broadly neutralizing antibody (bNAb) VCR01 against HIV — and mostly, it lost.

VCR01 only worked on HIV variants that 30% of participants had. But in those 30%, it was 75% effective at preventing HIV. Now you have Imbokodo, with its potential 25% activity against HIV, something that may have been a fluke. This, to Warren, requires a rethinking of the whole HIV vaccine enterprise while "doubling, tripling, quadrupling down" on the HIV prevention methods we know work, like pre-exposure prophylaxis (PrEP).

Dieffenbach agreed. To clinicians, Dieffenbach said the message of this HIV vaccine trial is flush with urgency: "Get your HIV-negative, at-risk people on PrEP tomorrow."

There are now two pills approved for HIV prevention, both of which have been found to be up to 99% effective when taken consistently. A third option, injectable cabotegravir (Vocabria), has been submitted to the US Food and Drug Administration for approval. The federal Ready, Set, PrEP program makes the pill available for free for those who qualify, and recently the Biden administration reaffirmed that, under the Affordable Care Act, insurance companies should cover all costs associated with PrEP, including lab work and exam visits.

But for the 157 women who participated in the trial at Gill's site in Masiphumelele, on the southwestern tip of South Africa, the trial was personal, said Jason Naidoo, community liaison officer at the Desmond Tutu HIV Foundation, which conducted a portion of the study. These were women whose parents, siblings, or children were living with HIV or had died from AIDS-defining illnesses, he said. Their lives were chaotic, traveling at a moment's notice to hometowns on the Eastern Cape, an 11-hour car ride away — longer by bus — for traditional prayers, funerals, and other important events.

Naidoo remembers arranging buses for the women to return for scheduled clinic visits, leaving the Eastern Cape in the afternoon and arriving in Masiphumelele in the early morning hours, just to keep the clinic appointment. Then, they'd turn around and return east.

They did this for 3 years, Naidoo said.

"The fact that these participants have stuck to this and been dedicated amidst all of the chaos talks about their commitment to actually having a vaccine for HIV," he said. "They know their own risk profile as young Black women in South Africa, and they understand the need for an intervention for the future generations."

"So you can understand the emotion and the sense of sadness, the disappointment —the incredible [dis]belief that this [the failure of the vaccine] could have happened, because the expectations are so, so high."

For Gill, who is lead investigator for Imbokodo in Masiphumelele, the weariness toward vaccines is back. Another trial is also underway for an HIV vaccine with another platform that was successful in COVID-19 — using messenger RNA (mRNA), like the Pfizer and Moderna COVID-19 vaccines did.

"I think we need to be careful," she said, "thinking that the mRNA vaccines are going to crack it."

Dieffenbach, Gill, and Naidoo have disclosed no relevant financial relationships. The study was funded by Janssen, a Johnson & Johnson company, with NIAID and the Bill and Melinda Gates Foundation.

Heather Boerner is a science journalist based in Pittsburgh. Her book, Positively Negative: Love, Pregnancy, and Science's Surprising Victory Over HIV, came out in 2014.

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