The vaccine prevented transmission of the herpes simplex 2 virus to nearly all mice that received the shot, said lead researcher Dr. Harvey Friedman, a professor and immunologist at the University of Pennsylvania's Perelman School of Medicine, in Philadelphia.
Researchers could not find any evidence of the herpes virus in 63 of 64 (98%) vaccinated mice that were then infected with herpes, he said.
"This degree of protection is better than any other vaccine candidates have achieved in mice," Friedman said. "We also had excellent protection in guinea pigs, which is another animal model used to evaluate genital herpes vaccine candidates."
There is no cure for herpes simplex virus 2, also called genital herpes, which is usually spread by vaginal, anal or oral sex. An estimated 14% of Americans between the ages of 15 and 49 are infected, researchers said in background notes.
Herpes can have dire consequences to human health. Genital herpes can triple or quadruple the risk of HIV infection. It can also cause a potentially deadly infection in newborns of mothers with herpes, the study authors said.
According to Dr. Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security, in Baltimore, "A vaccine could lessen the burden of this disease, decrease transmission, and prevent other sexually transmitted infections - including HIV - that are facilitated by HSV-2 infections."
Friedman said that the new vaccine takes a genetics-based approach to preventing herpes transmission. It combines the messenger RNA (mRNA) coding for three proteins found in the virus.
One of the proteins is required for the virus to enter cells and replicate, while the other two proteins help the virus evade detection by the immune system, Friedman explained.
The immune response to this vaccine is meant to teach the body how to defend against infection, creating antibodies that would easily target the virus and prevent it from infecting cells, he added.
Adalja said that the technology has the potential to "totally change the face of vaccine development in terms of speed, ease of manufacture and pursuit of novel targets. The promising results seen with this mRNA vaccine in mouse and guinea pig model will hopefully move into human clinical studies and also further the rise of mRNA vaccine technology."
Although the results are promising, human trials will be the real proof of the vaccine's effectiveness, the researchers stressed.
"Results in mice and guinea pigs have misled scientists in the past," Friedman said. "Our candidate vaccine looks superb, but no champagne until we find out if it protects people."
His team is in discussions with a biotechnology company to start human trials within a year in 50 to 100 people. The first trial would focus on the vaccine's safety and its ability to produce an immune response in humans.
"If those results look promising, we will then proceed to much larger studies in 2,000 to 5,000 people to evaluate whether the vaccine will protect people from acquiring genital herpes," Friedman said. "These studies from start to finish will take at least five years."
If the vaccine works, he said researchers envision recommending it for teenagers prior to the onset of sexual activity, much the same as the cancer-preventing human papillomavirus (HPV) vaccine is now administered.
A report on the experimental vaccine was published online Sept. 20 in the journal Science Immunology.
Sources: Harvey Friedman, M.D., professor and immunologist, University of Pennsylvania, Perelman School of Medicine, Philadelphia; Amesh Adalja, M.D., senior scholar, Johns Hopkins Center for Health Security, Baltimore; Sept. 20, 2019, Science Immunology, online
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