Scientists have blocked heroin addiction relapse in rats using a vaccine, according to U.S. News & World Report. They hope the vaccine will be ready for human testing later this year.

The vaccine allows the immune system to neutralize the effects of heroin and its byproducts, before the drug can reach the brain and cause a “high,” scientists from The Scripps Research Institute in California found. They say the vaccine might prevent a person addicted to heroin from overdosing on the drug.

Rats in the study were allowed unlimited access to heroin for several weeks, and then studied for four weeks while they received no heroin. Half of the rats were then given the vaccine, and all of the animals were reintroduced to heroin. “The rats who were not vaccinated relapsed quickly, the vaccinated rats stopped taking it,” said researcher George F. Koob. “In effect, what the vaccine does is prevent heroin from reaching the brain.”

The findings appear in the Proceedings of the National Academy of Sciences.

Last year, a researcher at the Walter Reed Army Institute of Research was awarded a grant from the National Institute on Drug Abuse to develop a vaccine that would treat heroin addiction and protect against HIV.