A vaccine may soon stop genital herpes, one of the most common sexually transmitted diseases. An experimental medicine has been successful on monkeys and actual guinea pigs, according to a study in PLOS Pathogens, and shows promise for the same result in humans.

Researchers from the University of Pennsylvania School of Medicine used a formula that simultaneously protects against the herpes simplex virus type 2, which causes genital herpes, in three ways by promoting different antibodies against three of its components. One of those “helps it break into host cells,” the university said in a statement. The other two pieces help it dodge your body’s defenses, enabling it to linger in someone for a long time — and genital herpes is known for recurring outbreaks. When administered once a month for three months, the vaccine stopped the virus from spreading between cells and raised the body’s immune response in animals.

Read: Is That a Pimple or Herpes?

“In essence, we’re stimulating the immune system to attack the virus and at the same time preventing the virus from using some of the tools it has to thwart that immune attack,” author Dr. Harvey Friedman, an infectious diseases professor, said in the Penn statement. “If the vaccine behaves like this in people, it would limit lesions to appearing only about one day in 100, and the virus would be potentially contagious only about two in every 1,000 days.”

According to the study, about 500 million people around the world carry the genital herpes virus. In addition to the painful sores in the genital area that accompany the infection, herpes also makes people more vulnerable to HIV, the sexually transmitted and bloodborne infection that leads to AIDS. This could be crucial in particular to people in sub-Saharan Africa, where infection rates of both genital herpes and HIV are higher than other places in the world: “An effective HSV-2 vaccine has been estimated to reduce the risk of HIV infection in sub-Saharan Africa by 30 to 40 percent.”

In the United States, one in six people between ages 14 and 49 has genital herpes, the Centers for Disease Control and Prevention estimate. Although type 2 of the virus is known as the one that causes genital herpes and the virus’ type 1, oral herpes, is most closely associated with cold sores around the mouth, type 1 can also cause genital herpes when spread during oral sex.

Source: Friedman HM, Awasthi S, Hook LM, et al. An HSV-2 Trivalent Vaccine Is Immunogenic in Rhesus Macaques and Highly Efficacious in Guinea Pigs. PLOS Pathogens. 2017.

See also:

Herpes Can Cause Hallucinations

STDs Are Spreading Faster Than Ever