Background: In recent decades, increasing rates of Chlamydia cases have contrasted with decreasing Chlamydia trachomatis seroprevalence rates and decreasing Chlamydia-associated complication rates. We elucidated the conflicting trends by studying incidence of repeated Chlamydia infections over time.
Methods: Chlamydia cases reported during 1995 to 2009 were identified in the Finnish National Infectious Diseases Registry. Trends of single and repeated diagnoses of Chlamydia infection were analyzed.
Results: Our study population comprised 147,148 individuals with a total of 177,138 genital chlamydial infections. The proportion of annual repeated diagnoses of genital infections increased among female and males from 4.9% to 7.3% and from 3.8% to 5.3%, respectively. In 2009, 24.8% of the females and 20.3% of the males had had an earlier Chlamydia infection ever during the follow-up time. Of all the repeated diagnoses, 34.1% occurred within 12 months. The highest rates of repeated infection diagnoses occurred in 25-year-old women (37.0%) and in 29-year-old men (30.9%) in a cohort of individuals born in 1979.
Conclusions: A gradual increase of repeated Chlamydia infections resulted in 43% increase in annual infections between 1996 and 2009. The result is supportive of the existing seroprevalence data suggesting that Chlamydia infection burden is not increasing in the whole population. The increasing infection rates in males, in particular, justify development of effective strategy in preventing reinfections and onward transmission.