Johnson & Johnson said on Tuesday its experimental vaccine failed to provide enough protection against HIV in sub-Saharan Africa to young women who are at high risk of being infected.

The mid-stage trial failure highlights the challenges of vaccine development, especially for HIV or human immunodeficiency virus, which has no approved vaccines.

“HIV is a unique and complex virus that has long posed unprecedented challenges for vaccine development because of its ability to attack, hijack and evade the human immune system,” Paul Stoffels, J&J’s chief scientific officer, said in a statement.

The study testing the vaccine included the participation of 2,600 women across five Southern African countries, where women and girls accounted for over 60% of all new HIV infections last year.

The trial of the vaccine, which is based on the adenovirus design which J&J’s COVID-19 vaccine also uses, was supported by the U.S. the National Institute of Allergy and Infectious Diseases and the Bill & Melinda Gates Foundation.

The study will not continue, based on the data, J&J said. Participants in the study will be informed about the results and told whether they received a placebo or the vaccine.

J&J said it is studying the safety and efficacy of a different composition of the vaccine regimen among men who have sex with men and transgender persons.