A discrepancy of Chlamydia trachomatis incidence and prevalence trends in Finland 1983–2003

A random sample of 7999 women with two consecutive pregnancies within five years was selected from the population of the Finnish Maternity Cohort (FMC) serum bank stratified by calendar year and age. C. trachomatis IgG antibodies were determined by a standard peptide-ELISA. The reported incidence rates of C. trachomatis infections based on case notifications were obtained from the National Registry of Infectious Diseases (NIDR).C. trachomatis seroprevalence rates decreased significantly from 1983 to 2003 both in women under 23 years of age (23.3% to 9.2%) and in women between 23–28-years of age (22.2% to 12.6%). However, seroconversion rates increased from 31 per 10000 person years in 1983–85 to 97 per 10000 person years in 2001–2003 (incidence rate ratio 3.2, 95% CI, 1.1–8.7) among the older age group. Seroconversion rate was highest (264) in 1983–1985 in the younger age-group, then declined and subsequently increased again (188) in 2001–2003. The incidence based on seroconversions was in agreement with the reported incidence rates in both age groups.C. trachomatis seroprevalence rate decreased during 1983–2003 among fertile-aged women in Finland. During the same time period incidence rates based both on seroconversions and reported laboratory notifications of diagnosed C. trachomatis infections increased. The discrepancy between the C. trachomatis incidence and seroprevalence trends warrants further studies.Chlamydia trachomatis is a major threat to the reproductive health of women [1]. National surveillance programs have largely failed to reduce the disease burden caused by C. trachomatis [2]. In the Nordic countries, incidence rates have increased between 1999 and 2005 [3-5]. In Finland, the incidence of reported C. trachomatis infections increased by 60% over the last 10 years, peaking in the age group of 15–24 [6]. Since most chlamydial infections are asymptomatic [7], true incidence may well be higher than that based on reports.In a population based study, we