Objectives: To examine how incidence of self-reported sexually transmitted infections (STIs) varies by gender and age, and the factors that influence this.
Methods: A longitudinal study of a cohort born in Dunedin, New Zealand in 1972/1973. They were questioned about STIs and sexual behavior at age 21, 26, and 32 years (1993–2005). Incidence rates were calculated over 3 age periods and compared using Poisson regression.
Results: Of the 1037 members of the original cohort, 92% or more of survivors completed the computer questionnaire at each age. Incidence rates of STIs from first coitus to age 21, age 21 to 26, and age 26 to 32, were 2.0, 3.2, and 2.0 per 100 person-years, respectively for men and 4.4, 3.0, and 1.4 per 100 person-years, respectively for women. After adjustment for sexual behavior, rates for men were elevated from age 21 to 26 compared with first coitus to 21 years of age [incidence rate ratio (IRR) = 1.9, 95% confidence interval (CI) 1.3 to 2.8), but not from age 26 to 32 (IRR = 1.1, 95% CI 0.70–1.9). For women, adjusted rates decreased with age; from 21 to 26 compared with first coitus to 21 (IRR = 0.79, 95% CI 0.56–1.1) and further from 26 to 32 (IRR = 0.39, 95% CI 0.27–0.57).
Conclusions: These unique data, comprising repeated assessment of reported behaviors and STIs in the same population, show that the period before age 21 is a time of special risk for STIs for women and of lower risk for men. The low risk among women aged 26 to 32 years after adjustment for sexual behavior warrants further investigation.