While an effective vaccine against HIV may still be a long way off, a new HIV prevention technique has proven remarkably effective at protecting women against the virus.

A single injection of a drug called cabotegravir every two months was so successful in preventing HIV in a clinical trial among women in sub-Saharan Africa that the study was wrapped up ahead of schedule.

The study, run by the HIV Prevention Trials Network, was looking at two forms of pre-exposure prophylaxis or (PrEP) aimed at women. PrEP is a technique of administering low doses of anti-AIDS drugs to people who are HIV negative as a way to protect them from infection. The study compared the effectiveness of the new long-acting injectable against the current form of PrEP, a daily pill of Truvada.

The findings were announced by the study's researchers on Monday.

"This is a major, major advance," said Dr. Anthony Fauci in a briefing. The director of the National Institute of Allergy and Infectious Diseases, which was involved in the study, Fauci has spent much of his career working on HIV/AIDS.

"This is a really good day for biomedical research," he said.

Until now, the main forms of HIV prevention for heterosexual women have been abstinence, condoms or PrEP with Truvada.

Truvada and cabotegravir are both antiretroviral drugs, medicines that have been incredibly effective in controlling HIV infections.

In the study of more than 3,000 women across 20 sites in sub-Saharan Africa, the cabotegravir injections were found to be 9 times more effective than Truvada.

"I don't think we can overemphasize the importance of this study," Fauci said in the briefing, announcing that the trial had been called to an end early because cabotegravir was clearly superior to the current form of PrEP.

"One of the stumbling blocks in our prevention [efforts against HIV] has been the inconsistency or lack of efficacy of pre-exposure prophylaxis in those who need it the most," Fauci said. "Namely young women, particularly those in southern Africa."

Last year an additional 1.7 million people were infected with HIV globally. Roughly 1 million of them were in Africa.

Rates of infection remain high in many African countries. In South Africa and Botswana nearly 20% of teens and adults between the ages of 15 and 49 are living with the virus. In Eswatini, also known as Swaziland, 27% of all adults are HIV-positive.

Pre-exposure prophylaxis trials — in which participants take a daily oral pill of Truvada — have been shown to be highly effective at preventing new HIV infections. But PrEP programs have been less effective in the real world.

Why was this new treatment so successful?It's easier to administer an injection every other month than it is to make sure someone takes a pill every day, says Sinead Delany-Moretlwe, one of the lead investigators on the new study and head of research at Wits Reproductive Health and HIV Institute at the University of the Witwatersrand in Johannesburg, South Africa. That gives cabotegravir an "adherence advantage" over Truvada, she says.

"What our results show is that cabotegravir was indeed superior to Truvada in preventing HIV infection," she says. Out of the 3,223 women in the study, half were getting Truvada and half were getting the injection. Three years after enrolling in the program, there were 34 HIV infections among women on Truvada versus just 4 in the group getting cabotegravir. This dramatic difference is striking in part because Truvada is already considered in the AIDS community to be a useful prevention tool.

Cabotegravir lingers in a person's body well past two months, raising the possibility of spacing injections out even further — perhaps just twice a year, some researchers have suggested.

Another study which came out earlier this year also found cabotegravir to be equally impressive in preventing HIV transmission among men who have sex with men.

Fauci isn't the only HIV specialist to hail this new finding. Leading infectious disease experts hailed the announcement of cabotegravir's success as "blockbuster" news and a potential "game-changer" for women.

Professor Monica Gandhi, a top HIV researcher at the University of San Francisco who was not involved in the new cabotegravir study in Africa, calls the results "exciting news."

"Women need options for HIV prevention, as women worldwide may not always be able to control their own risk of HIV infection," Gandhi told NPR. "Another option for HIV prevention is exciting and ground-breaking," she said.

Cabotegravir is already approved in Canada for treatment of HIV as part of a combination drug therapy sold under the brand name Cabenuva. ViiV Healthcare, which developed cabotegravir, plans to seek regulatory approval from the U.S. Food and Drug Administration for the drug both as pre-exposure prophylaxis and as a treatment in the first half of 2021 which could pave the way for its use in low-income countries.

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