Changes in the trend of sexually acquired chlamydia infections in Sweden and the role of testing: a time series analysis.

BACKGROUND We investigated notification trends of sexually acquired chlamydia (chlamydia) and its association with testing in Sweden prior to (1992-2004) and after (2009-2018) the discovery of a new variant of C. trachomatis (nvCT). METHODS We applied monthly time series analysis to study chlamydia trends and annual time series to study chlamydia rates adjusted for testing. We analyzed incidence nationally and by county group (based on able and unable to detect nvCT at time of discovery). RESULTS We present data on 606,000 cases of chlamydia and 9.9 million persons tested. We found a U-shaped chlamydia trend during 1992-2004 with an overall increase of 83.7% from 1996 onwards. The period 2009-2018 began with a stable trend at a high incidence level followed by a decrease of 19.7% during 2015-2018. Peaks were seen in autumn and through during winter and summer. Similar results were observed by groups of county, though with varying levels of increase and decrease in both time periods. Furthermore, increased testing volume was associated with increased chlamydia rates during the first period (P=0.019), but not the second period. CONCLUSIONS Our results showed that chlamydia trends during 2009-2018 were not driven by testing, as they were during 1992-2004. This suggests less biased notified chlamydia rates and thus possibly true decrease in chlamydia incidence rates. It is important to adjust case rates for testing intensity, and future research should target other potential factors influencing chlamydia rates.