The finding comes from a large study that collected daily genital swabs from nearly 500 people infected with herpes simplex virus type 2 (HSV-2), the genital herpes virus. Many (18%) thought they were uninfected, but found out they were herpes carriers when they underwent blood tests.
It's not too hard to find people who carry the virus, as 16% of Americans have HSV-2 infections. The vast majority -- between 75% and 90% -- don't know they are infected because they don't get, or don't notice, herpes sores on their genitals.
These asymptomatic herpes carriers shed infectious virus 10% of the 30 or more days they were in the study, report University of Washington researcher Anna Wald, MD, MPH, and colleagues. And nearly all the time, these people had no obvious sign of herpes infection while they were actively shedding virus.
"The primary concern of many HSV-2-seropositive persons is the risk of transmission to sexual partners; in our experience this is the main source of angst in patients with genital herpes," Wald and colleagues note in the April 13 issue of the Journal of the American Medical Association.
Asymptomatic-infected people shed herpes virus only about half as often as do people who have herpes symptoms. But when they are shedding virus, they shed just as much as people who have frequent symptoms (unless they are having an active herpes outbreak).
Wald and colleagues also found that:
- Men with genital herpes virus infection shed infectious virus just as often as women do.
- Men can shed infectious herpes virus through normal-appearing genital skin.
- Infection with HSV-1, the herpes virus that causes cold sores, does not make a person shed HSV-2 more or less often.
- It's not clear how much genital herpes virus it takes to infect a person, but evidence suggests only a "relatively moderate shedding episode" can infect a sex partner.
- People with eight or more genital herpes outbreaks per year shed infectious virus 31% of the time. People with one to seven outbreaks a year shed infectious virus 19% of the time.
- In the Wald study, white people shed virus more often than non-white people, but there were very few non-white people in the study, so the finding is questionable.
Because it's impossible to tell whether a sex partner is actively shedding virus, prevention hinges on knowing whether or not you are infected. You can find out via a simple blood test.
For those who know, or suspect, that they or their sex partner carries the genital herpes virus, each of these steps cuts the risk of transmission in half: