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Rise and fall of the new variant of Chlamydia trachomatis in Sweden: mathematical modelling study
  1. Joost H Smid1,
  2. Christian L Althaus1,
  3. Nicola Low1,
  4. Magnus Unemo2,
  5. Bjőrn Herrmann3
  1. 1Institute of Social and Preventive Medicine, University of Bern, Bern, Switzerland
  2. 2Department of Laboratory Medicine, Clinical Microbiology, Faculty of Medicine and Health, Örebro University, Örebro, Sweden
  3. 3Section of Clinical Bacteriology, Department of Medical Sciences, Uppsala University, Uppsala, Sweden
  1. Correspondence to Dr Christian L Althaus, Institute of Social and Preventive Medicine, University of Bern, Bern, Switzerland; christian.althaus{at}


Objectives A new variant of Chlamydia trachomatis (nvCT) was discovered in Sweden in 2006. The nvCT has a plasmid deletion, which escaped detection by two nucleic acid amplification tests (Abbott-Roche, AR), which were used in 14 of 21 Swedish counties. The objectives of this study were to assess when and where nvCT emerged in Sweden, the proportion of nvCT in each county and the role of a potential fitness difference between nvCT and co-circulating wild-type strains (wtCT).

Methods We used a compartmental mathematical model describing the spatial and temporal spread of nvCT and wtCT. We parameterised the model using sexual behaviour data and Swedish spatial and demographic data. We used Bayesian inference to fit the model to surveillance data about reported diagnoses of chlamydia infection in each county and data from four counties that assessed the proportion of nvCT in multiple years.

Results Model results indicated that nvCT emerged in central Sweden (Dalarna, Gävleborg, Västernorrland), reaching a proportion of 1% of prevalent CT infections in late 2002 or early 2003. The diagnostic selective advantage enabled rapid spread of nvCT in the presence of high treatment rates. After detection, the proportion of nvCT decreased from 30%–70% in AR counties and 5%–20% in counties that Becton Dickinson tests, to around 5% in 2015 in all counties. The decrease in nvCT was consistent with an estimated fitness cost of around 5% in transmissibility or 17% reduction in infectious duration.

Conclusions We reconstructed the course of a natural experiment in which a mutant strain of C. trachomatis spread across Sweden. Our modelling study provides support, for the first time, of a reduced transmissibility or infectious duration of nvCT. This mathematical model improved our understanding of the first nvCT epidemic in Sweden and can be adapted to investigate the impact of future diagnostic escape mutants.

  • Chlamydia trachomatis
  • Sweden
  • nucleic acid amplification test
  • mathematical model
  • basic reproduction number

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  • Handling editor Katy M E Turner

  • Twitter @c_althaus, @nicolamlow

  • Contributors BH and NL designed and coordinated the study. JHS with support from CLA and NL performed the modelling. All authors were involved in the analysis and interpretation of the results, and writing and approving of the final version of the manuscript.

  • Funding JHS received support from the Swiss National Science Foundation (grant number 160320). CLA received support from the LUSTRUM Programme of Research, funded by the National Institute for Health Research (NIHR) under its Programme Grants for Applied Research Programme (reference number RP-PG-0614–20009).

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement All data relevant to the study are included in the article or uploaded as supplementary information.

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