In a stunning development, an experimental vaccine successfully prevented HIV infection in half of monkeys that received it, a new study has found.

The monkeys were given the vaccine and then given high doses of an aggressive virus that is similar to HIV, according to a Daily Mail report.

It resulted in such amazingly positive results that Johnson & Johnson has begun a trial in 400 healthy volunteers in the United States, East Africa, South Africa, and Thailand, according to the report.

It comes eight years after Merck failed to provide good results in a 2007 trial, and it is the first time a major pharmaceutical company has sponsored an attempt to develop an HIV vaccine. HIV affects 35 million people worldwide, and it eventually leads to AIDS. It first began spreading 30 years ago, and 40 million people have died from it.

Treatment has been progressing with AIDS, but scientists believe that a vaccine is the best way to go to prevent it.

The studies involved a two-step vaccine that would prime the immune system by using a weakened version of the cold virus, and then sneaking in HIV genes into teh body. In the second phase, individuals would be injected with a purified HIV surface protein, which would provoke a strong immune response in the body.

It’s a similar strategy to what is begin attempted with the Ebola vaccine, which is in early-stage human trials.

The HIV vaccine trial in monkeys was particularly aggressive and was intended to test the limits of the vaccine by subjecting the animals to high doses of the aggressive virus, known as simian immunodeficiency virus, a cousin to HIV.