Objective: To determine the risk of sexual transmission of genital herpes simplex virus (HSV) in heterosexual couples.

Design: Prospective study of couples who were participants in a clinical trial. Each source partner had symptomatic, recurrent genital HSV, and each susceptible partner was without serologic or clinical evidence of genital herpes. Couples were followed for a median of 334 days.

Setting: Two university-based research clinics.

Patients: One hundred forty-four heterosexual couples were studied out of an initial enrollment of 214 couples.

Main outcome measures: Development of culture-proven HSV infection or type-specific antibodies in the susceptible partner.

Main results: Transmission occurred in 14 (9.7%) couples, including 11 (16.9%) of 65 couples with male and 3 (3.8%) of 79 with female source partners (P = 0.05). The annual rate of acquisition was higher (31.8%) in susceptible female partners who lacked antibodies to either HSV type 1 or 2 at entry compared with females with HSV type 1 antibodies at entry (9.1%). Couples avoiding transmission of HSV reported fewer days with genital lesions in source partners. Detailed histories were available at the time of transmission in 13 couples. In nine couples, transmission occurred when the source partner was reported to be asymptomatic and in four, it resulted from sexual contact at the time of prodrome (1 case) or within hours before lesions were first noticed by the source partner (3 cases).

Conclusions: Despite clear recognition of genital herpes in source partners, there was substantial risk for transmission; in 70% of patients, transmission appeared to result from sexual contact during periods of asymptomatic viral shedding. The risk for acquisition of HSV was higher in women than men, and previous HSV type 1 infection appeared to reduce the risk for acquisition of HSV type 2 infection among women.