A 2-stage trial testing a new and acclaimed HIV-prevention drug has shown almost unthinkable results of no new infections among a sample size of 3,200 participants.

Called PURPOSE 1, the aim of the first trial was testing a subcutaneous injection of the drug Lenacapavir given twice a year to people in a high-HIV-incidence country, which in this case was Uganda or South Africa.

The results were nothing short of extraordinary—100% efficacy, not a single young woman contracted HIV.

This was followed up by PURPOSE 2, which expanded the geographical area significantly to more countries on more continents, and expanded the pool of individuals from beyond just young women to men—and to those of all ages. 5,000 participants took part.

The result was the same: 99.9% reduction in infection rates.

Both were considered phase 3 clinical trials, and were conducted in a randomized, double-blinded protocol, but were not tested against a placebo. Instead, the Lenacapavir injections were compared to the current standard of HIV prevention—a pill called Truvada or Descovy taken daily.

These both were also found to prevent HIV transmission by 99.9% during development, but must be taken every day to achieve this level of protection. As anyone who’s tried to stick to a once-a-day pill regime long-term will agree, it’s not an easy thing to maintain month after month.

By contrast, the twice-yearly injections are much easier to adhere to, and they also come with the added benefit of removing the social stigma of being seen taking a daily pill and therefore at risk of HIV transmission. This can be particularly alleviating in high-HIV-prevalent countries where male homosexuality is illegal, such as Uganda.

Indeed the superiority of a twice-yearly injection was so clear that both PURPOSE trials were halted early over ethical reasons. A 52-week follow-up screened for HIV developments.

Lenacapavir was named by Science Magazine as the Breakthrough of the Year in 2024, and was approved by the FDA for use in humans under the brand name Yeztugo.

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It works to break down the HIVs capsid shell by binding to an “highly conserved” protein on the exterior. That means that no matter how many times or into what form the virus mutates, the exterior shell protein remains—presenting the perfect target for the drug.

In layman’s terms, the drug then works through the protein to disrupt the capsid shell, which the virus ‘takes down’ and ‘builds up’ several times during its lifecycle with perfect geometric precision. The disruption prevents the virus from completing its life cycle.

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Initial R&D, regulation compliance, and proof of efficacy and safety requirements mean that producing Lenacapavir has cost its developer, Gilead Sciences, an undisclosed total cost that would be reasonable to estimate at well over a billion dollars based on normal pharma development costs.

Gilead has nevertheless committed to providing the drug at cost in certain low-income regions and has licensed generic manufacturers to produce it for approximately $40 per year in 120 low and middle-income countries starting in 2027.

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