Background.  Herpes simplex virus infections type 1 (HSV-1) and type 2 (HSV-2) are common, but the epidemiology of HSV disease is changing.

Methods.  HSV-seronegative women, aged 18–30 years, who were in the control arm of the HERPEVAC Trial for Women were followed for 20 months for primary HSV infections.

Results.  Of the 3438 evaluable participants, 183 became infected with HSV: 127 (3.7%) with HSV-1 and 56 (1.6%) with HSV-2. The rate of infection for HSV-1 (2.5 per 100 person-years) was more than twice that for HSV-2 (1.1 per 100 person-years). Most infections (74% of HSV-1 and 63% of HSV-2) occurred without recognized signs or symptoms of herpes disease. The HSV-2 infection rate was 2.6 times higher in non-Hispanic black participants than in Hispanics and 5.5 times higher than in non-Hispanic whites (P < .001), while the HSV-1 infection rate was 1.7 times higher in non-Hispanic whites than non-Hispanic blacks. Younger participants (18–22 years) were more likely to acquire HSV-1 infections and less likely to develop recognized disease than older participants. Overall, 84% of recognized disease cases were genital. No differences were noted in the clinical manifestations of genital HSV-1 vs genital HSV-2 disease. The clinicians’ assessment that cases were caused by HSV was good when they assessed cases as clinically confirmed or unlikely (validated in 83% and 100% of cases, respectively).

Conclusions.  HSV-1 is now more common than HSV-2 as a cause of oral and genital mucosal infections in young women, but there are important age and race differences.