(Bloomberg) -- Gilead Sciences Inc.’s experimental twice-yearly shot prevented 100% of HIV cases in women and adolescent girls in Africa, the first successful big trial of what’s hoped to become a powerful new drug regimen for fending off the virus.

Gilead hopes its new long-acting shot lenacapavir will provide an easier-to-use option compared to other HIV prevention drugs that are either daily pills or must be injected every two months. The two yearly injections could be done at regularly scheduled doctor visits, making it easy for people to get.

HIV newly infects about 1.3 million people a year worldwide, and while current drug treatment is effective, researchers are seeking better ways to prevent the potentially lethal disease. While a vaccine hasn’t been developed, Gilead’s latest drug prevention regimen presents a potential option.

Gilead shares rose as much as 8.3% in New York, their biggest intraday gain since October 2022. Before Thursday, the stock was down 22% this year.

The study result was a “near best-case” scenario for the company, RBC Capital Markets analyst Brian Abrahams said in a note to clients. He said the preventive use of the drug could bring in $1.7 billion or more in annual sales. Lenacapavir is already approved under the brand name Sunlenca to treat HIV that’s resistant to other medicines, but it’s not a big seller for Gilead.

The trial involved about 5,300 women and female adolescents ages 16 to 25 in South Africa and Uganda, some of whom who received Gilead’s lenacapavir and others who received older once-daily drugs from Gilead, including Truvada or Descovy.  

No cases of HIV occurred among women in the trial who received the twice-yearly shot, Gilead said in a statement, significantly lower than the background rate of infections in those populations. The shot was also superior to Truvada.

The results were better than even Gilead had expected. No other HIV-prevention strategy “has had zero infections before” in a late-stage study, said Jared Baeten, a doctor who heads HIV development for Gilead. 

Gilead is conducting a second prevention trial of lenacapavir in men who have sex with men, as well as transgender women and transgender men. That trial, which also includes people in the US, is expected to yield results as soon as late 2024. If successful, Gilead expects to file for regulatory approval for preventive use of lenacapavir in the US and other countries. 

(Updates with comments from analyst in fifth paragraph and Gilead executive in eighth.)

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