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

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 currently on the market was so convincing the researchers decided to end their clinical trial of the drug early.

Women and girls accounted for about half of all new HIV infections in 2019, according to UNAIDS, a United Nations organization that leads the global 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. “That is why this study is so important. 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 freedom from having to hide or negotiate their medication use with their sexual partners, experts said.

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

And women will need easy access to the shots every eight weeks, which must be given by a medical professional.