Chlamydia trachomatis infection in Sweden : time trends, risk factors, and prevalence

Chlamydia trachomatis (chlamydia) infection is the most common bacterial sexually transmitted infection (STI), with an estimated 127 million new cases occurring every year worldwide. Due to the asymptomatic nature of the infection, individuals may carry it for a long time and transmit without knowing about it. Untreated chlamydia may lead to serious sequelae of the reproductive tract, causing pelvic inflammatory disease, chronic pain, ectopic pregnancy, and tubal factor infertility. Thus, early detection of infected individuals via testing (opportunistic testing), treatment, and partner notification may prevent further transmission of infection.

In this thesis, we aimed to gain an extended understanding of chlamydia epidemiology at an individual and population-based level. The thesis is based on the data from a cohort study in the urban STI clinic, as well as chlamydia cases and tests reported to the national infectious diseases register SmiNet-2 at the Public Health Agency of Sweden. We employed various methodologies to answer our study questions. In Studies I and II, we used data from a cohort study, which we analysed cross-sectionally. We found that being 20-24 years old, having 6 or more sexual partners during the previous 12 months, using alcohol before sex, reporting all type of sexual activities during the last sexual contact, and testing due to partner notification were independently statistically significantly associated with increased risk to test positive for chlamydia. Furthermore, we identified four groups (latent classes) of behaviour patterns among men and three among women in our cohort. The classes characterized by high-risk sexual behaviour were associated with statistically significantly increased 2-fold odds for lifetime repeated testing for chlamydia among men and women. Women in the high-risk behaviour class had also 2-fold increased odds to test repeatedly during the previous 12 months. This indicates that individuals at higher risk for chlamydia acquisition had adhered to the public health messages to test if at risk for infection. In Study III, we used time series analysis to explore how chlamydia trends changed over time, by comparing two periods: before and after the discovery of a new variant of Chlamydia trachomatis in Sweden. We analysed data nationally and by two types of counties, grouped according to their ability to identify new variant at the time of discovery. We also adjusted chlamydia trends to the testing intensity. We found that chlamydia trends were increasing since the mid-1990s up to 2004, as was testing, suggesting that chlamydia notification trends were driven by the testing. On the other hand, during 2009-2018 chlamydia trends were decreasing, despite increasing testing intensity, suggesting that chlamydia cases were not driven by testing, and most probably, these trends reflect true chlamydia incidence rates in the population. In Study IV, we continued to explore the reason for the decreasing chlamydia trends during 2009-2018 by estimating chlamydia prevalence via mathematical modelling. Indeed, we estimated a decrease in chlamydia prevalence among 15-29 year old men and women during this period, which supports our hypothesis.

In conclusion, the risk factors independently associated with chlamydia diagnosis are in line with previous studies. Furthermore, our results suggest that individuals in the high-risk sexual behaviour latent classes are more likely to test repeatedly for chlamydia, suggesting absorption of public health messages. Moreover, chlamydia trends were not driven by testing intensity during 2009-2018, suggesting a true decrease in chlamydia incidence rates in the population. By estimating the decrease in chlamydia prevalence during this period, we were able to support the hypothesis on the reason for observed declining chlamydia trends. Prevention work should be continued to reach asymptomatic individuals by testing and prevention messages. Further studies should investigate the role of other components of chlamydia control strategy on chlamydia trends to disentangle their input and plan for possible future alterations.

List of scientific papers

I. Veličko I, Ploner A, Sparén P, Marions L, Herrmann B, Kühlmann-Berenzon S. Sexual and testing behaviour associated with Chlamydia trachomatis infection: a cohort study in an STI clinic in Sweden. BMJ Open. 2016 Aug 26;6(8):e011312.
https://doi.org/10.1136/bmjopen-2016-011312

II. Veličko I, Ploner A, Marions L, Sparén P, Herrmann B, Kühlmann-Berenzon S. Patterns of sexual behaviour associated with repeated chlamydia testing and repeated chlamydia infection in men and women at urban STI-clinic in Stockholm: a latent class analysis. [Submitted]

III. Veličko I, Ploner A, Sparén P, Herrmann B, Marions L, Kühlmann-Berenzon S. Changes in the trend of sexually acquired chlamydia infections in Sweden and the role of testing: a time series analysis. Sexually Transmitted Diseases. October 27, 2020. [Accepted]
https://doi.org/10.1097/OLQ.0000000000001318

IV. Veličko I, Hansson D, Colombe S, Kühlmann-Berenzon S. Estimation of population-based Chlamydia trachomatis prevalence among 15-29 years old individuals in Sweden: a mathematical modelling study. [Manuscript]