People with asymptomatic genital herpes can still transmit the virus, a study finds.

This post has been corrected. See the note at the bottom for details.

Genital herpes is a common sexually transmitted disease, affecting about 16% of U.S. adults. A lot of people, however, don’t have outbreaks of painful blisters and don’t know they have the infection. Others know they have herpes but believe they can’t transmit it to a sexual partner unless they’re experiencing symptoms.

A study published Tuesday paints a far more complex portrait of genital herpes, also called herpes simplex virus type 2. researchers conducted one of the largest studies to date of people who test positive for herpes type 2. Some of the participants had occasional symptoms of herpes, and others were always asymptomatic. The 498 participants were asked to collect a genital swab each day for at least 30 days. The researchers, from the University of Washington and Fred Hutchinson Cancer Research Center, then looked at evidence of viral shedding -- the presence of actively replicating virus that can be transmitted to another person. They also noted whether the participant had any obvious signs of an outbreak.


Among 410 people who had symptoms, active virus was found on 20.1% of the testing days compared with 10.2% of the days in the 88 people who had herpes but were asymptomatic. Presence of the virus was detected at least once in 83.4% of the people with symptomatic infection and 68.2% of people with asymptomatic herpes.

What does it all mean? The authors say the study shows that people with herpes type 2 -- even those who don’t know they have it -- can transmit it to a sexual partner. The bulk of viral shedding occurs on days when people have no symptoms at all.

The study raises questions about the need for more blood testing to identify people who have herpes but don’t know it and counseling of patients about how to prevent infecting sexual partners. Condoms, daily use of the antiviral medication valacyclovir and disclosing one’s positive status are all effective ways of reducing transmission, researchers said. The study was published in the Journal of the American Medical Assn.

For the record: 2:48 p.m. April 14: An earlier version of the headline for this article incorrectly said, “Genital herpes can be transmitted even between outbreaks.” A corrected headline appears at the top of the article. Also, an earlier version of the accompanying photo caption incorrectly said, “Genital herpes can be transmitted between obvious outbreaks, study finds.” A corrected caption appears with the photo.

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