A new study from the UW Schools of Public Health and Medicine estimates the viral loads below which the herpes simplex virus-2 is unlikely to be transmitted.

The study was conducted with the Fred Hutchinson Cancer Research Center in Seattle and published in the Journal of the Royal Society. Mathematical models were used since it is impossible to directly measure genital viral load at the time of sexual activity. Researchers predicted that transmission is unlikely at viral loads of less than 10,000 copies of herpes simplex virus (HSV)-2 DNA. Antiviral therapies or immunotherapies that maintain viral load below that level could prevent most if not all transmissions.

HSV-2 affects one of every six adults in the US and more than 50 percent of persons in certain regions of sub-Saharan Africa. The virus can be transmitted when there are no visible lesions. Previous work has shown that antiviral therapy reduces the risk of transmission by 50 percent; condoms are also partly effective.

Senior author was Dr. Anna Wald, professor of epidemiology and of allergy and infectious diseases and director of the Virology Research Clinic at the University of Washington. Dr. Wald led a recent study showing that a new drug called pritelivir has shown effectiveness against HSV-2.

Dr. Youyi Fong, affiliate assistant professor of biostatistics, was a co-author of the latest study. Lead author was Dr. Joshua Schiffer, assistant professor of allergy and infectious diseases.