Reasons for the sharp increase of genital chlamydia infections reported in the first months of 2007 in Sweden.

After a continuous increase in the reported chlamydia incidence over the past 10 years in Sweden, the incidence decreased by 2% in 2006. A new genetic variant of Chlamydia trachomatis (nvCT) was discovered in Sweden in October 2006 that could not be detected by some of the commonly used diagnostic tests, which led to underreporting of chlamydia cases. This variant has also been called "swCT" by some authors. After the switch at the end of 2006 to other diagnostic tests that can detect nvCT, the reported incidence rose considerably (75 per 100,000 population) in the beginning of 2007. The objective of this study was to explore alternative explanations for this increase and to propose further action if needed. A data quality check was done in order to exclude double reporting and delayed reporting. To compare the incidence of chlamydia and the proportion of the population that was tested, we divided the Swedish counties into two groups, according to the diagnostic test used. We estimated the chlamydia incidence trend for January and February in the years from 2000 to 2005 by regression model, and predict the chlamydia incidence for the same period in 2006 and 2007. The age and sex distribution of the cases in January and February did not differ between the years 2000 to 2007. The proportion of tested people increased on average by 5% every year. If we assume that the percentage of the population that was tested had been 20% higher in 2007 than in 2006, the incidence predicted by the model for January and February 2007 is exactly the same as the incidence that was actually observed. The change of diagnostic test and an increase in the number of people tested, as well as the increase in the prevalence of CT have probably all contributed to the increased numbers of reported chlamydia cases in January and February 2007. These findings support the need for enhanced prevention campaigns in order to control spread of CT.