That is because improved hygiene in rich countries is lowering HSV-1 infection rates in childhood, leaving young people more at risk of catching it via oral sex when they become sexually active.

HSV-2 can increase the risk of catching and spreading HIV, the disease that causes AIDS. Little is known about any link between HSV-1 and HIV/AIDS, although it can lead to other serious complications such as encephalitis.

"We really need to accelerate the development of vaccines against herpes simplex virus, and if a vaccine designed to prevent HSV-2 infection also prevented HSV-1, it would have far reaching benefits," said Sami Gottlieb, a WHO medical officer.
Nathalie Broutet, also a WHO medical officer, said the U.S. National Institutes of Health and companies including GlaxoSmithKline Plc were involved in trials to determine whether a therapeutic or preventative vaccine was preferable.

Gottlieb said GSK had previously abandoned a vaccine trial after finding the product was not effective against HSV-2, although it did show some efficacy against HSV-1.

"That was interesting and promising and gave a proof of concept that these vaccines can be developed. 

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There's a lot of work ongoing and we're hopeful that we'll have an HSV vaccine in the future," she said.

Several phase-1 and phase-2 trials were underway, she said. Genocea Biosciences Inc recently dropped work on a pneumonia vaccine in favor of its more promising work on genital herpes. (Reporting by Tom Miles; Editing by Gareth Jones)

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