An annual injection designed to guard against Human Immunodeficiency Virus (HIV) has completed an important early safety trial, researchers report in The Lancet medical journal.

Lenacapavir stops the virus from replicating inside cells.

If future trials go well – now it has passed the first, Phase I, testing hurdle – it could become the longest-acting form of HIV prevention available.

Currently, people can take daily pills or sometimes have injections every eight weeks, for pre-exposure prophylaxis (PrEP), to reduce their risk.

PrEP tablets are highly effective but taking them every day can be difficult.

About 39.9 million people are living with HIV, 65% of them in the World Health Organization African Region, according to the most recent figures, for 2023.

And the WHO, Global Fund and Joint United Nations Programme on HIV and Aids (UNAids) are all working on strategies to end the HIV epidemic by 2030, which include improving access to medicines such as PrEP.

For the trial, 40 people without HIV were injected into the muscle with lenacapavir, with no major side effects or safety concerns.

And after 56 weeks, the medicine was still detectable in their bodies.

Future trials must include more diverse participants, the researchers told the 2025 Conference on Retroviruses and Opportunistic Infections.

But they added: “Yearly dosing of lenacapavir has the potential to further decrease current barriers to PrEP by increasing the uptake of, persistence on, and, therefore, scalability of PrEP.”

Richard Angell, from the Terrence Higgins Trust HIV charity, said: “PrEP as a daily pill has been a game-changer in our HIV response.

“The prospect of ‘safe’ annual injectable PrEP is as exciting as it is transformational.

“It’s great to see these early results that suggest injectable PrEP might be effective for up to 12 months.

“We need to get ready for its rollout now and fund sexual-health clinics to do so.”

Access to PrEP had been variable, Mr Angell said, creating inequalities.

“The oral pill is still not available in prisons, online or in community pharmacy,” he said.

“The Scottish Medicines Consortium has finally approved two-monthly PrEP injections for use in Scotland – but we are waiting for NICE [National Institute for Health and Care Excellence] approval for use in England.”