Hi again 22art22-ga, and thanks for your question.
You raise an important question regarding transmission of Herpes
Simplex Virus, Type I (HSV I). It is, unfortunately, a common myth
that HSV cannot be transmitted unless one has a symptomatic blister.
This is not true. Transmission during a symptomatic phase of the
disease is more efficient, because the viral titer (concentration) is
higher during this phase.
The basic answer to your question is about 1.7% of days throughout the
year, but there is variability and the answer depends on gender and
treatment, which are discussed below...
Most information on asymptomatic shedding in genital HSV has been
studied with HSV 2 rather than HSV 1. One very easy to read, complete
overview of HSV infection that looks at HSV shedding, transmission,
virus types, etc., is the following conference proceeding:
Corey L, Simmons A, Eds. The Medical Importance of Genital Herpes
Simplex Virus Infection. International Herpes Management Forum, 1997.
This 72 page reference has a long discussion regarding viral shedding,
starting on page 8 of the PDF file (page 6 of the article). The pages
I quote from here on will refer to the actual article (not the PDF
file). Please note that this article contains graphic medical
On page 11, there is a direct discussion of the question you pose,
which starts off by discussing the more common genital HSV-2:
"Transmission of HSV during periods of asymptomatic shedding has been
reported in several case studies,[11,12,19] but one of the first
studies systematically to examine asymptomatic shedding followed 27
women with recurrent genital herpes. Asymptomatic shedding was
detected in 80% of those followed for more than 50 days and overall
the virus was shed asymptomatically on 1% of days.
Another study found that 65% of women shed HSV on days without genital
lesions. Asymptomatic shedding occurred on a mean of 1.7% of days
sampled, although 11% of women with HSV-2 shed asymptomatically on
more than 5% of days (Figure 10). In this study, using viral
culture to detect HSV, asymptomatic shedding accounted for 32% of the
total time when viral shedding was
observed. The majority of clinical and asymptomatic episodes of viral
shedding lasted for 1 day, but 25% lasted 2 or more consecutive
Figure 10 on page 12 seems to directly answer your question, showing
for a group of women, the percentage of days on which they had
asymptomatic shedding of virus for both HSV-1 and HSV-2.
The authors go on to compare shedding in HSV-1 and HSV-2 on pages 12-13:
"The frequency of asymptomatic shedding of HSV from genital sites is
significantly lower in women who have acquired HSV-1 compared with
women who acquired HSV-2 or both HSV-1 and HSV-2. Koelle et al(1992)
demonstrated in a prospective study that the frequency of asymptomatic
shedding detected by culture was more common during the first 12
months after first-episode genital HSV-2 infection (3.3?4.3% of days)
than in the 12 months after first-episode HSV-1 infection (1.2% of
days [Table 4])."
On page 13, an additional study is cited that gives a frequency of
asymptomatic shedding on 1.7% of days (women) for HSV-1:
"Another study has documented culture-proven asymptomatic shedding on
2.0% of days in women with genital HSV-2 and 0.7% of days in a cohort
of women with genital HSV-1.21 The rate of asymptomatic shedding was
similar in HSV-2 seropositive women and those who were seropositive
for both HSV-1 and -2."
These results are based on viral cultures. More recent data from PCR
(polymerase chain reaction) show that these rates are likely even
higher, but more work is needed to determine the clinical significance
of these findings. You can read more on pages 13-14 in the above
Interestingly, there is less data available for asymptomatic shedding in men:
"There are much less data on the frequency and pattern of asymptomatic
shedding in men despite the fact that the efficacy of transmission
appears to be greater from men to women.[18,28] The evidence available
suggests that asymptomatic shedding is similar in men and women, being
documented from the urethra, urine and normally appearing penile
skin.[29,30] HSV has also been isolated in the semen in the absence of
discernible lesions.[31,32] The rate of asymptomatic shedding in men
is about 2.2% and the penile skin is the most common site of
asymptomatic reactivation. Asymptomatic shedding in men also
shows clustering and can occur before and after a symptomatic
Treatment and shedding
The article above includes a lengthy discussion of the effects of
treatment (aciclovir) on asymptomatic shedding. The results are
summarized in Figure 12 on page 15:
"Of the 34 women enrolled, 26 completed both treatment periods and
cultures were obtained on more than 80% of study days. In those
completing the first treatment period, preventative aciclovir
treatment was associated with a 94% decrease in the frequency of
asymptomatic shedding; days with asymptomatic shedding were reduced
from 5.8% on placebo to 0.37% on aciclovir
(P< 0.001 [Figure 12]). Seventy-three per cent (19/26) of patients
shed HSV asymptomatically whilst receiving placebo compared with 19%
(5/26) during aciclovir therapy. This study did not quantify the
amount of virus and the implications for transmission are unknown."
To summarize, aciclovir therapy appears to significantly reduce the
rate of asymptomatic viral shedding and may, therefore, decrease
transmission rates, which is a current topic of clinical research.
Figure 13 (page 16) shows rates of viral shedding using the PCR detection method.
The fact that HSV-1 and -2 can be spread during asymptomatic shedding
has some important implications:
"There is growing evidence that HSV can be transmitted in the absence
of clinical signs
and symptoms. The virus can be shed from the genital tract when there
are no lesions;
two studies by Mertz and colleagues found that up to 70% of genital HSV infections
were transmitted when the source partner was asymptomatic.[3,4]"
Most patients are unaware of the potential consequences:
"Most patients are unaware of the fact that they could be infectious
when they have no symptoms. The incidence of asymptomatic virus
shedding is known to be lower in HSV-1 infection compared with HSV-2
infection and therefore the typing of HSV infection could be important
from a prognostic point of view (see Chapter 2)."
There is also the concern of asymptomatic shedding resulting in
perinatal transmission during delivery (page 51):
"The source of HSV in perinatal infections is most often the birth
canal. At birth, as the neonate passes through the birth canal, it
comes into contact with infected secretions; the site of entry for the
virus is usually the eye or nasopharynx. There can be different sites
of HSV shedding in the birth canal but their relative importance is
unknown. It has been suggested that cervicovaginal shedding poses
a greater risk than shedding from the vulva because the neonate is in
close contact with the vaginal mucosa for several hours but is only in
light and transient contact with the vulva during delivery."
Up to 70% of neonates infected via the perinatal route received the
virus from mothers who were experiencing asymptomatic shedding (see
page 51). The effects of treatment on asymptomatic shedding and
C-sections, etc., can be seen in Table 9 (page 58).
The figure on page 61 shows rates of asymptomatic shedding in HIV
positive and negative individuals. Below this figure is a discussion
of CD4 cell count and shedding.
A very accessible review article looking at genital herpes infections
was published by a group in London in 1997:
Brugha R. Keersmaekers K. Renton A. Meheus A. Genital herpes
infection: a review. [Review] [127 refs] [Journal Article. Review.
Review, Tutorial] International Journal of Epidemiology.
26(4):698-709, 1997 Aug.
Fortunately, a free full text copy of the article is available by
following the link from the page above.
The authors have the following to say with regard to transmission of
HSV, with reference to what (little) is known about shedding rates:
"Herpes simplex virus is transmitted by symptomatic lesions and
through asymptomatic viral shedding, the former being more efficient
because lesions have higher viral titres.[13,15] Transmission of HSV-2
may be more efficient from men to women (4/13 women in steady
relationships with HSV-2 positive men seroconverted over 3 years,
compared with no seroconversions among 16 seronegative men with HSV-2
positive female partners). This may partly be due to the higher
rate of disease recurrences in men which may make them more
The authors also discuss the association between HIV and asymptomatic
"Asymptomatic shedding occurs in both men and women but is more easily
detected in women, mainly from the cervix and vulva. It is more common
in the first year after first-episode disease, in HSV-2 was first
diagnosed. Therefore, disease due to primary genital herpes
infection is currently rare in HIV-infected people. However, it is
likely to increase in frequency as individuals with congenitally
acquired HIV infection survive into adult life. Being HIV seropositive
has been shown to be a strong risk factor for subsequent first-episode
and recurrent-episode genital herpes disease in those who are also
HSV-2 positive, independently of other known behavioural risk and
protective factors. infections, and for one week after symptomatic
recurrences.[12,15,49?51] The contribution of asymptomatic viral
shedding to the genital transmission of HSV in the population has not
been quantified, but is believed to account for most
transmissions. In 50?90% of transmissions, the source contact is
unaware of being infected.[19,25,28,52?58] Therefore effective
control of genital herpes transmission at the population level will
not be possible through interventions targeted only at those with
I hope this information was useful. Please feel free to request any
clarification prior to rating.