As with almost all sexually transmitted infections, women are more susceptible to acquiring genital HSV-2 than men. On an annual basis, without the use of antivirals or condoms, the transmission risk of HSV-2 from infected male to female is approximately 8-11%. This is believed to be due to the increased exposure of mucosal tissue to potential infection sites. Transmission risk from infected female to male is approximately 4-5% annually. Suppressive antiviral therapy reduces these risks by 50%. Antivirals also help prevent the development of symptomatic HSV in infection scenarios—meaning the infected partner will be seropositive but symptom free—by about 50%. Condom use also reduces the transmission risk significantly. Condom use is much more effective at preventing male to female transmission than vice-versa. The effects of combining antiviral and condom use is roughly additive, thus resulting in approximately a 75% combined reduction in annual transmission risk. These figures reflect experiences with subjects having frequently recurring genital herpes (>6 recurrences per year). Subjects with low recurrence rates and those with no clinical manifestations were excluded from these studies. Previous HSV-1 infection appears to reduce the risk for acquisition of HSV-2 infection among women by a factor of 3.
However, asymptomatic carriers of the HSV-2 virus are still contagious. In many infections, the first symptom a person will have of their own infection is the horizontal transmission to a sexual partner or the vertical transmission of neonatal herpes to a newborn at term. Since most asymptomatic individuals are unaware of their infection, they are considered at high risk for spreading HSV.
In October 2011, it was reported that the anti-HIV drug tenofovir, when used topically in a microbicidal vaginal gel, prevented herpes virus infections.
Barrier methods
Condoms offer moderate protection against HSV-2 in both men and women, with consistent condom users having a 30% lower risk of HSV-2 acquisition compared with those who never use condoms. A female condom can provide greater protection than the male condom, as it covers the labia. The virus cannot pass through a synthetic condom, but a male condom's effectiveness is limited because herpes ulcers may appear on areas not covered by the male condom. Neither type of condom prevents contact with the scrotum, anus, buttocks, or upper thighs, areas which may come in contact with ulcers or genital secretions during sexual activity. Protection against herpes simplex depends on the site of the ulcer; therefore if ulcers appear on areas not covered by condoms, abstaining from sexual activity until the ulcers are fully healed is one way to limit risk of transmission. The risk is not eliminated, however, as viral shedding capable of transmitting infection may still occur while the infected partner is asymptomatic. The use of condoms or dental dams also limits the transmission of herpes from the genitals of one partner to the mouth of the other (or vice versa) during oral sex. When one partner has a herpes simplex infection and the other does not, the use of antiviral medication, such as valaciclovir, in conjunction with a condom further decreases the chances of transmission to the uninfected partner. Topical microbicides which contain chemicals that directly inactivate the virus and block viral entry are being investigated.
Vaccine
Vaccines for HSV are undergoing trials. Once developed, they may be used to help with prevention or minimize initial infections as well as treatment for existing infections.
Antivirals
Antivirals may reduce asymptomatic shedding; it is believed asymptomatic genital HSV-2 viral shedding occurs on 20% of days per year in patients not undergoing antiviral treatment, versus 10% of days while on antiviral therapy.
Pregnancy
The risk of transmission from mother to baby is highest if the mother becomes infected at around the time of delivery (30% to 60%), but the risk falls to 3% if it is a recurrent infection, and is less than 1% if there are no visible lesions. To prevent neonatal infections, seronegative women are recommended to avoid unprotected oral-genital contact with an HSV-1 seropositive partner and conventional sex with a partner having a genital infection during the last trimester of pregnancy. A seronegative mother who contracts HSV at this time has up to a 57% chance of conveying the infection to her baby during childbirth, since insufficient time will have occurred for the generation and transfer of protective maternal antibodies before the birth of the child, whereas a woman seropositive for both HSV-1 and HSV-2 has around a 1-3% chance of transmitting infection to her infant. Women who are seropositive for only one type of HSV are only half as likely to transmit HSV as infected seronegative mothers. Mothers infected with HSV are advised to avoid procedures that would cause trauma to the infant during birth (e.g. fetal scalp electrodes, forceps, and vacuum extractors) and, should lesions be present, to elect caesarean section to reduce exposure of the child to infected secretions in the birth canal. The use of antiviral treatments, such as acyclovir, given from the 36th week of pregnancy, limits HSV recurrence and shedding during childbirth, thereby reducing the need for caesarean section.
Acyclovir is the recommended antiviral for herpes suppressive therapy during the last months of pregnancy. The use of valaciclovir and famciclovir, while potentially improving compliance have less well determined safety in pregnancy.