New Zealand women participating in a longitudinal cohort study reported a greater incidence of STD diagnoses before age 21 than between ages 26 and 32; for men, by contrast, incidence was higher at ages 21–26 than earlier.1 In addition, the reported incidence of any STD by age 21 was nearly three times as high among women as it was among men, and a gender difference was apparent for both bacterial and viral infections. Between ages 21 and 32, however, women and men reported similar STD rates.
The cohort consisted of men and women who were born in Dunedin in 1972–1973 and were enrolled in a multidisciplinary health study at age three. Cohort members were followed up every two years until they were 15, and then at ages 18, 21, 26 and 32. At age 21, the cohort was roughly representative of 21-year-olds nationwide. The last three follow-up assessments, administered through a computerized questionnaire, elicited information about participants' sexual behavior and sexual health. At each of these assessments, 400–500 men and a similar number of women were sexually active and answered questions on STDs; researchers used data from these participants to calculate STD incidence rates, which they compared in a series of regression analyses.
By the time they were 21, about eight in 10 of both men and women with sexual experience had had more than one partner; at age 26, a substantial majority of both reported having had multiple partners since the previous survey, and at age 32, three in four men and half of women said that they had had multiple partners since the previous assessment. More than half of respondents of each gender reported always or usually using condoms when they were 21; in subsequent study intervals, the proportions were about 20-–40%. Concurrent partnerships were reported by 10% or fewer of respondents at each assessment, and most participants said that their last partner had had no more partners than they had.
At age 21, 9% of men and 18% of women said that they had had at least one STD diagnosed by a clinician. Fourteen percent of each reported having had an STD between ages 21 and 26, and 9% of each said they had had one between ages 26 and 32. Chlamydia, genital warts and herpes were the most commonly reported STDs by both men and women at each survey.
Men's reported incidence of STDs translated to a rate of 2.0 infections per 100 person-years by 21 years of age, 3.2 per 100 at ages 21–26 and 2.0 per 100 at ages 26–32. Analyses adjusting for participants' number of sexual partners indicated that the rate in the middle period was almost twice that of the earliest period (incidence rate ratio, 1.9). In separate examinations of bacterial and viral STDs, the analysts found no significant differences by age in men's reported rates of infection.
Among women, STDs occurred at a rate of 4.4 per 100 person-years between first intercourse and 21 years of age, 3.0 per 100 between ages 21 and 26, and 1.4 per 100 at ages 26–32. In analyses that adjusted for both number of partners and condom use in the last 12 months, the rate was significantly lower in the last interval than in the first (incidence rate ratio, 0.4); the rates in the first two intervals were statistically indistinguishable. Rates of bacterial STDs were significantly reduced at ages 21–26 (0.5) and 26–32 (0.4), and the rate of viral STDs was reduced at ages 26–32 (0.4).
Women's reported STD incidence by age 21 was significantly higher than men's (incidence rate ratio, 2.6 in analyses adjusting for number of sexual partners). No gender difference was apparent in the later study intervals. The pattern was the same for both bacterial and viral diseases (incidence rate ratios, 4.3 and 2.0, respectively, at the age 21 assessment).
The researchers acknowledge several limitations of their study—among them, the use of participants' own reports of STD diagnoses and deficiencies in available measures of sexual behavior and partners' risk characteristics. Nevertheless, they point out that the longitudinal data set, with its repeated measures of sexual behavior and STDs, provided them with a "unique opportunity" to examine age effects on reports of STD diagnoses and to directly compare patterns in these effects by gender. The results, they add, "should be generalizable" to the United States and other developed countries with similar STD–specific incidence rates. Given the study's strengths, they conclude that the period between first intercourse and age 21 is "a time of special risk" for STDs among women, but of reduced risk among men.—D. Hollander
1. Paul C et al., Longitudinal study of self-reported sexually transmitted infection incidence by gender and age up to age thirty-two years, Sexually Transmitted Diseases, 2009, 36(2):63–69.