A single shot given every two months has proved to be more effective than a daily pill at preventing HIV in women, researchers reported, an advance that medical experts hailed as groundbreaking.

The finding that the long-acting drug would prevent HIV in six doses taken over a year instead of the 365 required for the prevention pill on the market was so convincing the researchers decided to end their clinical trial of the drug early. "It's a game changer for women," said Dr. Sigal Yawetz, an expert on women with HIV at Brigham and Women's Hospital in Boston who was not involved in the trial.

Women and girls accounted for about half of all new HIV infections in 2019, said UNAIDS, a United Nations organization that leads the fight against HIV and AIDS. In sub-Saharan Africa, 5 in 6 new infections among adolescents ages 15-19 are among girls.

"If we're going to get to the end of the epidemic, we have to do something to stem the tide of infection in those women," said Dr. Kimberly Smith, head of research and development at ViiV Healthcare, which manufactures the injection. "It gives a new, incredibly effective option for women."

Women have had only one approved option for pre-exposure prophylaxis, or PrEP, a course of drugs taken to prevent contracting HIV: the daily pill Truvada, made by Gilead Sciences. (A second pill also made by Gilead, called Descovy, was approved in October 2019, but only for men and transgender women.)

Many women struggled to take the pill regularly, undercutting its usefulness. Not having to take a daily pill would afford them more freedom, experts said. "We've been waiting for a PrEP strategy that would work in women, and I think we have one now — so it's very exciting," Yawetz said. "I'd like women to have as many options as they can that are affordable and safe."

But the injected drug will need to be affordable in resource-poor nations. Even in the U.S., Truvada's price kept it out of reach of many who would have benefited from the drug. (Generic versions are now available.)

And women will need easy access to the shots every eight weeks, which must be given by a medical professional. During the clinical trial, because of restrictions introduced during the pandemic, "there was a scramble to make sure that these women could continue to get their injections," Smith said. The trial researchers sent transportation for the women and managed to retain 98% of the participants, she said.

The randomized, double-blind clinical trial was conducted by the HIV Prevention Trials Network, an international collaborative funded by the National Institutes of Health. The trial compared the injected drug, called cabotegravir, with Truvada in 3,223 participants in 20 sites across seven countries. Of the women who received Truvada, 34 became infected with HIV during the trial, compared with just four of the women receiving the injections; two of those four women had stopped taking the injections, Smith said.

"I am extremely excited about the findings of this study," said Dr. Monica Gandhi, an HIV expert at the University of California, San Francisco.