The latest attempt at an HIV vaccine has failed, as researchers announced Monday they have stopped giving the experimental shots in a major study.

The study had enrolled more than 5,400 people since 2016 in South Africa, a country with one of the world’s highest HIV rates. Last month, monitors checked how the study was going and found 129 HIV infections had occurred among the vaccine recipients compared with 123 among those given a dummy shot, according to the U.S. National Institutes of Health.

“An HIV vaccine is essential to end the global pandemic and we hoped this vaccine candidate would work. Regrettably, it does not,” said NIH infectious diseases chief Dr. Anthony Fauci.

There were no safety concerns, but NIH, which sponsored the study, agreed that vaccinations should stop.

The experimental shot was based on the only vaccine ever shown to offer even modest protection against HIV, one that was deemed 31% effective in Thailand. That wasn’t good enough for real-world use but gave scientists a starting point. They beefed up the shot and adapted it to the HIV subtype that’s common in southern Africa.

Two other large studies, in several countries, are under way testing a different approach to a possible HIV vaccine.