Herpes simplex virus (HSV) infections are endemic, but the clinical characteristics of newly acquired HSV type 1 (HSV-1) and HSV type 2 (HSV-2) infections in adults have not been rigorously defined.

We monitored 2393 sexually active HSV-2–seronegative persons for clinical and serologic evidence of new HSV infection. Of the participants, 1508 were seropositive for HSV-1 and 885 were seronegative. Charts were reviewed in a blinded manner for classification of those with genitourinary or oropharyngeal symptoms. Charts were also reviewed for all 174 persons with HSV seroconversion.

The rates of new HSV-1 and HSV-2 infections were 1.6 and 5.1 cases per 100 person-years, respectively. Of the 155 new HSV-2 infections, 57 (37 percent) were symptomatic, 47 of which (82 percent) were correctly diagnosed at presentation. Among the 74 patients given a clinical diagnosis of genital HSV-2 infection during the study, 60 were given a correct diagnosis and 14 were given an incorrect diagnosis, for a ratio of true positive results to false positive results of 4:1. Among the 98 persons with asymptomatic HSV-2 seroconversion, 15 percent had genital lesions at some time during follow-up. Women were more likely than men to acquire HSV-2 (P<0.01) and to have symptomatic infection. Previous HSV-1 infection did not reduce the rate of HSV-2 infection, but it did increase the likelihood of asymptomatic seroconversion, as compared with symptomatic seroconversion, by a factor of 2.6 (P<0.001). Of the 19 new HSV-1 infections, 12 were symptomatic. The rates of symptomatic genital HSV-1 infection and oropharyngeal HSV-1 infection were the same (0.5 case per 100 person-years).

Nearly 40 percent of newly acquired HSV-2 infections and nearly two thirds of new HSV-1 infections are symptomatic. Among sexually active adults, new genital HSV-1 infections are as common as new oropharyngeal HSV-1 infections.