Chlamydia 2019-2021

Chlamydia 2019-2021

Data from the Danish Microbiology Database

As in the previous years, the data needed for the preparation of the present report covering 2019-2021 were collected via the Danish Microbiology Database (MiBa). The data comprise all positive and negative test results for oculo-genital infection caused by Chlamydia trachomatis at all departments of clinical microbiology (DCMs). The data include civil registration numbers, and disease episodes can therefore be delimited to the individual level. A new disease episode is defined as an interval of more than 42 days between two positive test episodes. Several negative test results within a single year are counted as a single negative testing episode. For patients with a valid civil registration number, information about the municipal code and thereby the area and region is collected from the Danish civil registration system. In the current annual report covering 2019-2021, numbers from 2018 are often provided to allow for comparison. Until the end of 2015, monitoring was based on the laboratory notification system, as shown in Table 1. The differences between the two methods are described in EPI-NEWS 34/2016.

Other data

The extent of chlamydia testing performed in private laboratories that market themselves via the internet remains unknown. Such data are not included in this report. When rectal chlamydia is established or when lymphogranuloma venereum (LGV; EPI-NEWS 20/2008) is suspected, some DCMs and STI clinics submit material to Statens Serum Institut (SSI) for relevant specialised testing.

Chlamydia incidence

In 2021, a total of 272,809 persons were tested for chlamydia (275,760 in 2018), and the number of detected cases was 36,632 (33,415 in 2018). Since 2010, an increase has been observed in the number of persons tested, but 2020 recorded a decline, probably due to the COVID-19 pandemic. Furthermore, the number of detected chlamydia cases declined in 2020 compared with 2019, Table 1. Not all tested persons had their sex and municipality stated. Therefore, different positive rates may occur in the below tables.


Figure 1 presents the number of tested persons and the number of chlamydia cases per month from 2019 to 2021. In the months of March, April, May, August, September and October 2020, the number of tested persons and detected cases declined compared with 2019. In the months of November and December, the number of tests made was lower than in 2019, whereas the number of detected chlamydia cases was higher than in 2019. In the months of January, February and June, the number of tests and the number of detected cases were higher than in 2019. The numbers for 2021 are largely in line with the numbers for 2019 apart from January and February when the number of tests and chlamydia cases were lower than in 2020. Compared with 2019, the positive rate in 2020 remained unchanged in the months of January, February, April and August, but increased in all of the remaining months of the year.


Table 2 divides the numbers by sex and age groups and presents the number of chlamydia cases and the incidence per 105. The incidence of detected cases increased from 2018 to 2021. Compared with 2019, a decline was observed in 2020 for both sexes. The group of 15-29-year-olds comprised the largest share of the detected cases and has remained stable at around 81% for men and 89% for women since 2017. In 2021, men comprised 42% of the diagnosed cases. This share has followed a stable, increasing trend since 1994 when the share was 23%. Compared with 2018, the incidence for men and for women alike increased in the period until 2021, except for the decline observed in 2020.


Among the 15-29-year-olds, the incidence of detected chlamydia cases increased from 2018 to 2021, except for the decline observed in 2020. For women, the incidence was very high among 19-year-olds in all years from 2018 to 2021. For men, the highest incidence was observed among 20- and 21-year-olds. As in previous years, the female incidence was higher than the male incidence in the 15-29-year age group.


Geographical distribution of chlamydia cases

From 2018 to 2021, the national incidence of laboratory-detected chlamydia cases (per 105 inhabitants) increased for men and women alike.

For men, the incidence remained unchanged in Copenhagen subs. but declined in North Zealand, East Zealand and on the Island of Bornholm.

For women, corresponding incidences were observed in East Zealand, West and South Zealand and on the island of Bornholm. In the remaining Danish areas, the incidence recorded for both men and women followed an increasing trend. In 2020 compared with 2019, the incidence declined overall and in most areas, although the following exceptions were observed: Among men, an increase was observed in North Zealand and on the island of Bornholm. Furthermore, an increase was recorded for women in North Zealand, East Jutland and on the island of Funen. The incidence rate ratio between men and women increased for Denmark overall in 2021 and was 0.73 compared with 0.69 in 2018. Notably, a decline was observed in East Zealand from 0.76 to 0.70.



Among patients who had chlamydia detected, 91% of the samples were submitted by general practitioners or medical specialists and 9% by hospital departments (2018 and 2019: 89% and 11%, 2020: 90% and 10%). All cases were detected using nucleic acid amplification techniques (NAT). In 2021, chlamydia was detected in urine samples in 14,760 cases (2018: 13,296, 2019: 14,343, 2020: 13,857), i.e. 40% of all cases, which is in line with the numbers recorded from 2018 to 2020. A total of 89% of the positive urine samples were from men. Since 2018, this figure has increased by one percentage point annually. Urine was the sampling material used in 85% of men with chlamydia. This figure has remained practically unchanged for the years stated.

Chlamydia in children

Chlamydia was diagnosed in 182 children under 15 years of age (145 in 2018), including 120 aged 1-14 years (80 in 2018). In the 10-14-year age group, 14 chlamydia cases were seen in boys and 102 in girls (eight and 69, respectively, in 2018). The incidence per 105 among 10-14-year-olds was eight for boys (five in 2018) and 63 for girls (42 in 2018). The figures for 2018-2021 can be deduced from Table 2. Probably sexually transmitted chlamydia was detected in 102 girls, including 90 14-year-olds. Among these tests, 87 findings were made by urogenital swabs and one also by a rectal swab, whereas two were found by eye swabs. Additionally, nine 13-year-olds tested positive by urogenital swabs and one by a throat swab. Two 12-year-old girls had urogenital chlamydia.

Among boys, nine 14-year-olds and two 13-year-olds had chlamydia detected in the urine/urethra, whereas two 14-year-olds and one 11-year-old tested positive by eye swabbing. In 2021, chlamydia was detected in ocular samples from 55 children < 1 year, among whom 47 were less than one month old (in 2018, this figure was 54 of 62 children). One of these children also tested positive by ocular and tracheal swabbing. A three-month-old child tested positive by throat swab.

Age groups and sexes

Generally, a decline was observed in the incidence of tested men and women from 2018 to 2021, Table 4. However, an increase was observed in the testing incidence for both sexes in the age groups 15-19 years and 20-24 years, and for females aged < 1 year and 10-14 years. Overall, the incidence of tested men (2,706 per 105) was considerably lower than the share of tested women (6,570 per 105). This applied in all age groups, except for men < 1 year and 50+ years. In 2020, the total incidence and the incidences for all age groups for men and women separately were lower than those recorded in 2019. The share of tested men declined by 14% (413 per 105); and for women, the share declined by 7% (459 per 105), compared with 2019. From 2018 to 2021, the positive rate among the tested persons increased for men and women alike, by 2.7 and 0.7 percentage points, respectively. The greatest increase was observed in the 15-19-year age group, where the 2021 positive percentage was 36% for men and 26% for women. Furthermore, it should be noted that a considerable increase was recorded in the positive rate for the 10-14-year age group for both sexes.

Notably, both sexes recorded a considerably lower number of tested persons in 2020, but the positive percentage was considerably higher than in 2019.


Areas and sexes

Overall and in most Danish areas, the number of men and women (per 105 inhabitants) tested in 2021 declined compared with 2018, and a large decline was observed in 2020. Across Denmark, the share of tested women is more than twice as high as the share of tested men. At the national level, the positive rate increased for both sexes from 2018 to 2021; by 20% for men and 11% for women.

For women in the City of Copenhagen and East Zealand, the positive rate remained unchanged, whereas it followed a slightly declining trend in West and South Zealand.


Testing of men for rectal chlamydia

The incidence of men who underwent rectal chlamydia testing increased from 153 in 2018 to 232 in 2021. The incidence increased in all parts of Denmark, except for East Zealand and Funen. In the same period, the number of detected chlamydia cases increased by 46%, from 889 to 1,294 cases, whereas the positive rate remained practically unchanged (20% and 19%, respectively). In particular, a considerable increase in the number of tests and chlamydia cases was observed in North Jutland. Despite these differences between the various Danish areas, 2020 observed a decline in the number of tested men, whereas the number of detected chlamydia cases and the positive rate increased compared with 2019.


Lymphogranuloma venereum

The SSI performed specific LGV testing by NAAT (nucleic acid amplification test) for 1,236 patients (858 men, 374 women and four persons for whom the sex had not been stated). The number of ano-rectal samples was 1,048 (762 men, 282 women; and in four cases, the sex was not stated). Since 2018, the number of tested men has increased as has the share of ano-rectal swabs among men. LGV testing is performed for 59% of men with ano-rectal chlamydia, which is a six percentage point increase compared with 2018. However, the incidence of detected LGV has declined drastically since 2019. Thus, LGV was detected in only 16 men (2%) in 14 ano-rectal samples and two samples for which the sampling location was not stated.


This report is also described in EPI-NEWS 47/2022.