The original studies included 5,384 participants, of whom 419 (7.8%) acquired HSV-2. Of these 419 cases, 188 (44.9%) were excluded because they acquired the infection during their first visit and therefore had no control period data. Thirty additional participants were excluded as either the date of HSV-2 detection (n=4), or sexual behavior over the risk period (n=26) was unknown. 10 more participants were excluded to due to consistent reports of more than 7 sexual acts per week. Thus, 191 participants (45.6% of those who acquired) were included in our case-crossover analysis. The median age, gender, STI history, and method of HSV-2 detection were similar among included and excluded participants; however, race, sexual orientation (for men) and source study differed between the two groups ().
Baseline Characteristics of Those Who Acquired HSV-2 During Follow-Up, by Inclusion or Exclusion in the Case-Crossover Analysis
Participants were 14 to 53 years of age (median: 25.0; interquartile range (IQR): 21.0, 32.6); 54.5% were men. 51.3% were white, 40.8% black, 4.7% Hispanic, and 3.1% were of other races or ethnicities. 86.5% of the men were heterosexual; 13.5% were men who have sex with men (MSM). 56.5% of participants were HSV-1 seropositive at baseline and 46.1% reported having ever had a sexually transmitted infection (STI). 94.8% of HSV-2 infections were detected using serologic assays and the remainder by viral culture ().
Per-person follow-up time ranged from 97 to 748 days (median: 299; IQR: 184, 382) for a total of 57,966 person-days of follow-up. The number of study visits per person ranged from 2 to 13 (median: 4; IQR: 3, 7). The median time between visits was 56 days (IQR: 32, 77). A total of 952 study visits were included, with 264 visits (27.7%) contributing to the case periods, and 688 visits (72.3%) contributing to control periods.
Participants reported from 0 to 80 sexual partners during follow-up, with 43.5% (n=73) reporting only one partner during follow-up, 1.8% (n=3) reporting no partners, 14.9% (n=25) reporting two partners and 39.9% (n=67) reporting 3 or more partners during follow-up; data on number of partners were missing for 23 (12.0%) subjects. Most (56.0%) participants reported no new partners; while 13.1%, 6.8% and 24.1% reported one, two, and three or more new partners, respectively, during follow-up.
11,742 sexual acts were reported: 4,380 during case periods and 7,362 during control periods. The median number of acts reported per study visit was 7 (IQR: 2, 16), with 10 acts per visit during case periods (IQR: 3, 21) and 6 acts per visit during control periods (IQR: 2, 14). The median rate of sexual activity was 1.5 acts per week (IQR: 0.6, 2.8), with 1.7 acts per week during case periods (IQR: 0.7, 3.0) and 1.5 acts per week during control periods (IQR: 0.5, 2.7).
Condoms were used in an estimated 29.3% of sexual acts, including 24.3% of case acts and 32.3% of control acts. Summarizing a person’s condom use over all case periods and over all control periods separately, we compared each individual’s category of condom use in the case and control periods. Of the 181 participants who reported at least one sexual act in both the case and control periods, 104 (57.5%) remained in the same category of condom use during both the case and control periods; 19 (10.5%) were in a category of more frequent condom use during the case period, relative to the control period; and 58 (32.0%) were in a category of less frequent condom use during the case period (Stuart-Maxwell test of marginal homogeneity P<0.001) ().
Frequency of Condom Use During the Case and Control Periods for the 181 Participants Who Reported at Least One Sexual Act During Follow-Up
We found a 3.6% increase in the odds of HSV-2 acquisition with each unprotected act (OR=1.036; 95% CI: 1.021, 1.052). No significant increase in the odds of acquisition was detected for protected acts (OR=1.008; 95% CI: 0.987, 1.030). Thus, the estimated odds of acquiring HSV-2 were significantly lower with a protected act than with an unprotected act (Wald test P=0.029) (). Similarly, the model based on the categorized condom use estimated a 3.6% increase in the odds of HSV-2 acquisition with every act when condoms were never used (OR=1.036; 95% CI: 1.021, 1.052), a 2.7% increase in the odds of acquisition with each act when condoms were sometimes used (OR=1.027; 95% CI: 1.010, 1.044), and no significant increase in risk when condoms were always used (OR=0.989; 95% CI: 0.957, 1.023) (). Neither HSV-1 status, nor the number of partners, nor the number of new partners were explanatory or altered other associations in multivariate analyses.
Odds Ratios Reflecting the Change in Odds of HSV-2 Acquisition With Each Unprotected Act and Protected Act as Estimated by the All-Visits, Stratified All-Visits, Exposure Duration-Matched, and GEE Models
Table 3b Odds Ratios Reflecting the Change in Odds of HSV-2 Acquisition With Each Act During Periods in Which Condoms Were Never Used (Never-Period Acts), Sometimes Used (Sometimes-Period Acts), or Always Used (Always-Period Acts), as Estimated by the All-Visit (more ...)
Gender did not significantly modify the effect on HSV-2 acquisition of either the number of protected acts (P=0.34) or the number of unprotected acts (P=0.48). However, a difference was noted by discordant couple status (). While neither subgroup demonstrated any change in odds of acquisition with protected acts (P=0.49), the effect of unprotected acts on acquisition was lower in discordant couples (OR=0.992) relative to the rest of the cohort (OR=1.050; P=0.004 for the interaction).
To ascertain that our results were not influenced by the varying duration of case and control periods, we repeated the analysis using the expected number of sexual acts based on exposure duration matching. Results were similar when case and control periods of equal duration were established, with a 2.2% increase in the odds of HSV-2 acquisition associated with each unprotected act (OR=1.022; 95% CI: 1.003, 1.042) and no significant increase in risk detected for protected acts (OR=0.987; 95% CI: 0.954, 1.020); the odds of acquisition were significantly lower with protected than unprotected acts (P=0.043). The results of these subgroup analyses and the results of the all-visits and exposure duration-matched models are summarized in .
Figure 2 Forest plot summarizing odds ratios and 95% confidence intervals for change in odds of herpes simplex virus type 2 (HSV-2) acquisition associated with each type of act. The height of each box is proportional to the number of participants included in that (more ...)
Next, we performed an analysis of the full cohort, including 5,324 participants who did and did not acquire HSV-2 during the follow up. 39 participants (0.7%) were excluded due to reports of more than 7 sexual acts per week, and 21 (0.4%) were excluded due to missing data. We found a 1.3% increase in the odds of HSV-2 acquisition with each unprotected act (OR=1.013, 95%CI: 1.008, 1.018), and no significant change in the odds of acquisition with protected acts (OR=0.998; 95% CI: 0.988, 1.008)(). Thus, with each unprotected act the odds of acquisition were observed to increase 1.3% in the cohort analysis; a fraction of the 3.6% increase detected by the case-crossover analysis.