A study shows that a twice-yearly shot was 100% effective in preventing new HIV infections in women in Africa.

Twice-yearly shots used to treat AIDS were 100% effective in preventing new infections in women, according to study results published Wednesday.

There were no infections in the young women and girls that got the shots in a study of about 5,000 in South Africa and Uganda, researchers reported. In a group given daily prevention pills, roughly 2% ended up catching HIV from infected sex partners.

“To see this level of protection is stunning,” said Salim Abdool Karim of the injections. He is director of an AIDS research center in Durban, South Africa, who was not part of the research.

The results in women were published Wednesday in the New England Journal of Medicine and discussed at an AIDS conference in Munich. Gilead paid for the study and some of the researchers are company employees. Because of the surprisingly encouraging results, the study was stopped early and all participants were offered the shots, also known as lenacapavir.