Scientists have struggled with creating a vaccine for HIV/AIDS for decades now, as the virus’ job is solely to neutralize and debilitate the immune system. But results from a new study conducted by researchers at Johnson & Johnson show we may be soon be able to prevent this deadly disease from spreading. Six out of 12 monkeys in which a vaccine was tested not only resisted the virus, but also produced antibodies against it.

“Despite great progress in HIV treatments, HIV remains one of the greatest global health threats of our time, with millions continuing to be infected each year,” said Dr. Paul Stoffels, chief scientific officer and worldwide chairman of pharmaceuticals for Johnson & Johnson. “Our ultimate goal is to develop a vaccine that prevents HIV in the first place.”

Producing an HIV vaccine has been so difficult not only because the virus kills the immune system’s T cells, which are supposed to mount an attack against invading pathogens, but also because it employs a number of tactics to evade the immune system. The virus can also mutate incredibly quickly, making it difficult for immune cells to identify it, and it’s able to acquire proteins in the body that mask it from an immune response. The vaccine currently being tested may counter all these factors, though.

For the study, the researchers used a two-pronged approach. They first gave 12 rhesus monkeys an injection with a modified version of the common cold, called adenovirus 26, which was meant to prime their immune systems — it also contained genetic bits of the monkey version of HIV, known as simian immunodeficiency virus (SIV). Once this injection initiated an immune response, the monkeys were given another injection with proteins from the virus that normally help it stay hidden. Together, the injections worked to jumpstart the immune system and help it identify and fight large doses of the virus in six of the 12 monkeys. Some also began producing antibodies to the virus. And when the researchers tested the vaccine with a combined form of SIV and HIV, known as SHIV, 40 percent of the monkeys became protected.

SIV isn’t identical to HIV, however, they’re similar enough to develop vaccines with, lead author Dr. Dan Barouch of the Ragon Institute at Harvard, MIT, and Massachusetts General Hospital, told NBC News. And the findings are promising enough that Johnson & Johnson has begun enrolling 400 human volunteers to begin phase 1 trials.

“I do think that their results are impressive,” Dr. Mary Marovich, director of the vaccine research program at the National Institute for Allergies and Infectious Diseases, told NBC. “Even protecting half of the people who are exposed to the virus would be a major accomplishment. It could ultimately end the epidemic when you use in combination with other measures.”

Source: Barouch D, Alter G, Broge T, et al. Protective efficacy of adenovirus-protein vaccines against SIV challenges in rhesus monkeys. Science. 2015.