Epidemiology and determinants of reemerging bacterial sexually transmitted infections (STIs) and emerging STIs in Europe

Summary

In this scoping review, we offer a comprehensive understanding of the current and recent epidemiology, challenges, and emerging issues related to bacterial sexually transmitted infections (STIs) in the WHO European Region. We endeavour in collating data from both EU/EEA and non- EU/EEA countries, thereby giving a complete picture of the region which highlights the higher notification rates in Northern and Western countries than other regions, likely due to differences in testing, access to testing, and surveillance capacity. We provide an up-to-date review on the current knowledge of determinants and persistent inequities in key populations as well as the use of molecular epidemiology for identifying transmission networks in gonorrhoea and syphilis, and detecting chlamydia mutations that evade molecular diagnosis. Finally, we explore the emerging STIs in the region and the evolving transmission routes of food and waterborne diseases into sexual transmission. Our findings call for harmonized STI surveillance systems, proactive strategies, and policies to address social factors, and staying vigilant for emerging STIs.

Key messages.

Bacterial STIs notifications across the WHO European Region

• •Our review includes epidemiological data from 49 countries in the WHO European Region, encompassing reports from both EU/EEA countries and non- EU/EEA countries.
• •Considerable regional variations exist in bacterial STI notifications among countries in the WHO European Region. Countries in Northern and Western Europe have higher notification rates per 100000 population compared to other regions. These variations can be attributed to differences in surveillance systems performance, STI testing policies, access to testing, and the availability of sensitive diagnostic techniques and algorithms.
• •Temporal trends over the past decade show a slow increase in the case notification rate for Chlamydia trachomatis (CT), alongside a considerable increase for Neisseria gonorrhoeae (NG) and syphilis in EU/EEA countries, as opposed to decreasing trends in non- EU/EEA countries.
• •In 2021 in the EU/EEA, CT cases were mainly among 15-24-year-olds (60%), NG predominantly affected those aged 15–34 (66%), and syphilis cases were more common in individuals over 35 (55%). CT notifications were high among heterosexuals of both genders, NG and syphilis were more common in men, mostly in men who have sex with men.

STIs in key populations and people in vulnerable situations

• •A combination of factors/determinants, including biological factors, sexual behaviours, social connections, and structural barriers, contribute to the spread of HIV/STIs particularly among key populations.
• •Men who have sex with men in Europe have demonstrated a substantial increase in bacterial STIs over the past decade. While data on transgender and gender diverse individuals remain limited, transgender women exhibit a high epidemiological risk for STIs.
• •Displaced populations such as migrants and refugees experience sub-optimal access to health and prevention services, and sex workers have both limited access and are highly susceptible to STIs due to unsafe working conditions and criminalisation of sex work.
• •The challenges for STI management observed from epidemics like COVID-19 should serve as lessons informing the development of effective interventions to mitigating their impact and enhance future public health preparedness.

Insights from molecular epidemiology

• •NG cases, as revealed by molecular epidemiology, are predominantly acquired locally, and distinct NG transmission networks exist among men who have sex with men and among heterosexual populations.
• •T. pallidum's slow single nucleotide polymorphism acquisition limits molecular studies on transmission; however, combination of genomic data with meta-data revealed distinct syphilis transmission networks for men who have sex with men and heterosexual men.
• •In CT, molecular epidemiology has detected emerging variants like the nvCT (2006) and F-nvCT (2019) that initially evaded NAAT detection platforms in use at that time, resulting in undiagnosed infections and the subsequent national or regional spread of the variant.

Emerging STIs and emerging routes of sexual transmission for infectious pathogens

• •Zoonotic origins of sexually transmissible pathogens, such as HIV and Monkeypox virus, demonstrate the potential for pathogens to cross species barriers and adapt to human hosts, leading to widespread transmission and epidemics.
• •Shigella and Hepatitis A virus, traditionally transmitted through food and waterborne routes, have recently established sexual transmission routes, primarily among men who have sex with men.
• •Other emerging pathogens like Zika virus and Ebola virus can be transmitted via genital fluids, while Hepatitis C virus can be transmitted through contact with blood during sex.
• •Biological agents with the potential of being sexually transmitted can vary across regions and over time. Monitoring these potential emerging STIs is also crucial.

Search strategy and selection criteria.

This is a non-systematic narrative review. References for this review were identified through searches of PubMed, Embase and Science Direct with the search terms (“gonorrhoea” OR “Neisseria gonorrhoeae”), OR (“chlamydia” OR “Chlamydia trachomatis”), OR (“syphilis” OR “Treponema pallidum), OR (“sexually transmitted infection” OR “STI”), AND (“Europe” OR “epidemiology” OR “incidence” OR “prevalence” OR “notifications” OR “surveillance” OR “key populations” OR “men who have sex with men” OR “transgender” OR “migrants” OR “sex workers” OR “molecular epidemiology” OR “emerging”). The publication year was limited from 1995 until July 2023. We did not use language restrictions. Articles were also identified through searches of the authors’ own files. The final reference list was generated based on originality and relevance to the broad scope of this review.

Introduction

Sexually transmitted infections (STIs) pose a significant public health burden in the World Health Organization (WHO) European Region.¹ In this narrative review, our aim is to examine the current epidemiological trends and multilevel determinants of (re-)emerging STIs in both the European Union/European Economic Area (EU/EEA) and non- EU/EEA countries, focussing on bacterial STIs such as Chlamydia trachomatis (CT), Neisseria gonorrhoeae (NG), and Treponema pallidum (syphilis) infections. Our aim is to shed light on the current epidemiological situation to identify opportunities for reducing the burden of these infections and improving sexual health outcomes across the region.

The paper is part of a series of articles that cover, besides STIs epidemiology in Europe, subjects such as STI prophylaxis and vaccines, effective HIV combination prevention,² the role of online STI testing services and self-sampling,³ and antimicrobial resistance in STIs.⁴

Regional variations in notification rates

The global prevalence of bacterial STIs among individuals aged 15–49 years is annually estimated by the WHO using data from regions with robust case-based surveillance systems and population-based studies. In 2020, the overall world prevalence was 3.2% [2.7–3.9%] for CT, 0.7% [0.5–1.1%] for NG, and 0.6% [0.5–0.7%] for syphilis.⁵ The WHO European Region exhibited the lowest STI prevalence among all WHO regions for both genders, with values of 2.7% [2.2–3.5] for CT, 0.3% [0.1–0.5] for NG, and 0.11% [0.09–0.13] for syphilis. Additionally, lymphogranuloma venereum (LGV, CT serovar L1-L3) has been associated with several outbreaks in Europe; the infection, although notifiable, is considerably underreported.^6,7

We conducted a search for STI surveillance records in 53 countries within the WHO European Region (Table 1). We retrieved data from The European Surveillance System⁸ for 26 of 27 EU Member States (except for Austria), the United Kingdom and two EEA countries; from a WHO Regional Office for Europe report⁹ for 11 non-EU/EEA countries; and from national reports for Andorra¹⁰, Austria,¹¹ Switzerland,¹² the Republic of Moldova,¹³ the Russian Federation,¹⁴ Kyrgyzstan,¹⁵ Bosnia-Herzegovina,¹⁶ Montenegro,¹⁷ Israel.²⁷ Data sources could not be identified for four countries (Monaco, San Marino, Turkmenistan and Türkiye). Liechtenstein which is a EEA country started to report to the European Surveillance system in 2020.

Table 1.

Notification rates^a per 100,000 population for chlamydia, gonorrhoea, and syphilis in countries of the WHO European region, 2019.

Countries in the WHO European Regionb	Chlamydia	Gonorrhoea	Syphilis	Data source
EU/EEA Countries grouped by un geoscheme regionsd
Northern Europe
Denmark	614.53	38.06	6.22	SAID8
Estonia	80.31	5.89	2.79	SAID
Finland	293.24	10.96	4.55	SAID
Iceland	502.81	34.17	10.64	SAID
Ireland	182.72	57.32	15.19	SAID
Latvia	65.05	6.88	3.91	SAID
Lithuania	8.9	2	4.2	SAID/LMH18 (syphilis)
Norway	533.88	31.98	3.85	SAID
Sweden	340.01	31.72	4.21	SAID
United Kingdom	388.47	116.05	13.11	SAID
Western Europe
Austria	n/a	18.03	6.53	SA11
Belgium	72.01	22.89	14.51	SAID
France	35.40	5.61	1.68	BSP19 (CT)/SAID
Germany	n/a	n/a	9.55	SAID
Luxemburg	7.17	3.91	8.14	SAID
Monacoc	n/a	n/a	n/a	n/a
Netherlands	104.52	39.84	8.49	SAID
Switzerlandc	143.2	45.3	8.4	OFSP12
Southern Europe
Andorrac	12.3	8.6	2.5	GA10
Cyprus	0.11	0.23	3.54	SAID
Croatia	3.68	0.98	0.69	SAID
Greece	0.58	1.87	4.13	SAID
Italy	1.85	1.36	3.05	SAID
Malta	64.84	32.62	19.25	SAID
Portugal	7.66	10.98	4.66	SAID
San Marinoc	n/a	n/a	n/a	n/a
Slovenia	19.08	10.72	2.6	SAID
Spain	38.54	21.79	10.4	SAID
Eastern Europe
Bulgaria	1.73	0.31	6.86	SAID
Czech Republic	n/a	15.47	5.92	SAID
Hungary	9.34	13.97	8.06	SAID
Poland	1.1	0.74	4.28	SAID
Romania	0.07	0.17	2.78	SAID
Slovakia	14.31	6.77	5.1	SAID
Non- EU/EEA Countries grouped by un geoscheme regionse
Eastern Europe
Belarus	43.3	8.1	4.3	Barbaric et al.9/NSCRB20
Republic of Moldova	71.7	25	54.2	BSM13
Russian Federation	n/a	7.7	15	FSSS14
Ukraine	31.6	7.4	5.7	Barbaric et al./SSSU21/Krotik et al.22
Southern Europe
Albania	n/a	0.3	2.4	Barbaric et al.
Bosnia-Herzegovina	3.6	0.3	0.8	PIH16
Montenegro	3.05	0.48	0.32	SOM17
North Macedonia	4.6	0.67	0.4	Barbaric (CT, syphilis), SAID (NG)
Serbia	10	1.54	2.3	Barbaric (CT, syphilis), SAID (NG)
Central Asia
Kazakhstan	13.5	10.5	18.8	Barbaric et al./BNS23
Kyrgyzstan	19.25	2.25	4.8	ODPK15
Tajikistan	n/a	2.7	4	Barbaric et al.
Turkmenistan	n/a	n/a	n/a	n/a
Uzbekistan	n/a	10.7	8.7	Barbaric et al.
Western Asia
Armenia	26.3	9.8	3.7	Barbaric et al.9/SCRA24
Azerbaijan	19	3	8.2	Barbaric et al./TSSCRA25
Georgia	39	18.5	26.5	Barbaric et al./NSOG26
Israel	15.50	5.85	5.61	MHI27
Turkey	n/a	n/a	n/a	n/a

^aRegarding the methods used to calculate notification rates, data for EU/EEA countries were extracted from the individual country notification rates in the ECDC Annual Epidemiological Reports for 2019,28, 29, 30 with a detailed description of the calculation methods available elsewhere.³¹ In short, the numerator comprises reported cases, while the denominator consists of population data obtained from Eurostat as of January 1st of each year. For non- EU/EEA countries our primary approach involved utilizing notification rates per 100,000 population as reported in the official national documents. In cases where this information was not available, we considered the absolute number of notified cases as the numerator, and the population of the respective year as the denominator.

^bThe table includes data for 53 countries within the WHO European Region which were divided into two groups: the EU/EEA category and the non-EU/EEA category, and further categorized according to the United Nations geoscheme regions.

^cCountries that were part of the WHO European Region but were not in the EU/EEA region in 2019 (the UK was an EU member state in 2019). For the purpose of this review Switzerland and the European microstates—Andorra, Monaco, and San Marino—that are part of the WHO European Region but were not EU/EEA members in 2019 were allocated to their respective UN Geoscheme. Liechtenstein, an EEA member state, started to report to the European Surveillance system in 2020.

^dData from 29 EU/EEA countries in 2019 were extracted from the Surveillance Atlas of Infectious Diseases (SAID),⁸ hosted by the European Centre for Disease Prevention and Control (ECDC). In addition to SAID, national statistical reports were utilized for CT in France (BSP)¹⁹ and for syphilis in Lithuania (LMH)¹⁸ Data for 3 countries: Andorra (GA),¹⁰ Austria (SA)¹¹ and Switzerland (OFSP)¹² were extracted from their respective national statistical reports. We could not find data for the years 2019 (or 2018) for 2 countries: Monaco and San Marino.

^eData from 11 countries in the non- EU/EEA group were obtained from an assessment conducted by the WHO Regional Office for Europe (WHO/Europe), which included data extracted from the WHO Communicable Diseases Annual Reporting Forms (Babaric et al.)⁹ and complemented with national reports (NSCRB,²⁰ SSSU,²² BNS,²³ SCRA,²⁴ TSSCRA²⁵). The data corresponds to the year 2019, except for Albania, North Macedonia, Serbia and Tajikistan, which corresponds to the year 2018. Data from 6 other non- EU/EEA countries was obtained from national statistical reports, including notification rates for CT, NG and syphilis from Moldova (BSM),¹³ the Russian Federation (FSSS),¹⁴ Kyrgyzstan (ODPK),¹⁵ Bosnia-Herzegovina (PIH),¹⁶ Montenegro (SOM),¹⁷ and Israel (MHI)²⁷ The data for Bosnia and Herzegovina are only from the Republic of Srpska. We could not find data for the years 2019 (or 2018) for 2 countries—Turkmenistan and Turkey. CT reports were not found or not reported in Albania, Austria, Germany, Czechia, the Russian Federation, Tajikistan and Uzbekistan. NG reports were not found for Germany.

There are considerable regional variations in the bacterial STI notification rates among countries in the WHO European Region. For the purpose of comparison and considering the main data sources, we grouped countries as EU/EEA⁸ and non- EU/EEA,⁹ and further categorized according to the regions of the United Nations geoscheme (including European regions and Central and Western Asia). Countries with data originating from national reports were added to the respective geoscheme regions (Fig. 1, panel A). We primarily centred on 2019 data, available for a substantial number of countries.8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27

Fig. 1.

Geographical areas and case notification rates per 100,000 population ofChlamydia trachomatis(CT),Neisseria gonorrhoeae(NG),and Treponema pallidum(syphilis) infections in the WHO European Region in 2019. Legend. Panel A: Countries grouped by EU/EEA and non-EU/EEA and subgrouped by UN geoscheme regions (including European regions and Central and Western Asia). Panel B: Notification Rates per 100,000 population of Chlamydia trachomatis infections. Panel C: Notification Rates per 100,000 population of Neisseria gonorrhoeae infections. Panel D: Notification Rates per 100,000 population of Treponema pallidum infections. The UK was an EU member state in 2019. Switzerland and the European microstates – Andorra, Monaco, and San Marino – that are part of the WHO European Region but were not EU/EEA members in 2019 were allocated to their respective UN Geoscheme region.

In 2019, EU/EEA countries in Northern Europe had higher notification rates of CT, NG, and syphilis, than EU/EEA countries in other regions and non- EU/EEA countries. For CT, the majority of EU/EEA countries in Northern Europe had CT notification rates of >300 per 100,000 population (Table 1, Fig. 1 panel B), while EU/EEA countries in other regions reported rates ranging from 0.07 to 143.2 per 100,000.²⁸ Epidemiological figures in non- EU/EEA countries, showed notification rates of CT ranging from 3.1 to 71.7/100,000, with the highest rates observed in the Republic of Moldova (71.7), Belarus (43.3), Georgia (39.0), and Ukraine (31.6).⁹

The overall case notification rate of NG infections in EU/EEA countries in 2019 was 31.3/100,000 with an outlying high rate recorded in the UK (116.1) (Table 1, Fig. 1 panel C).²⁹ Historically, the incidence of NG in non-EU/EEA countries in Eastern Europe has been higher than the rest of the WHO European Region with estimates in some countries higher than 40/100,000,³² but the reported notification rates in 2019 were not as high (ranging from 7.4 to 25), and even lower in the rest of non-EU/EEA countries (0.3–10.5), with the exception of Georgia (18.5).⁹

Regarding syphilis, high rates (>8 per 100,000 population) were observed both in Northern Europe countries (e.g., Ireland [15.1], UK [13.1]), Western Europe (e.g., Netherlands [104.5], Belgium [72.0]), Southern Europe countries (e.g., Spain [10.4], Malta [19.2]),³⁰ and some non-EU/EEA countries of Eastern Europe (e.g., the Russian Federation [15]) and Central and Western Asia (e.g., Georgia [26.5], Kazakhstan [18.8]) (Table 1, Fig. 1 panel D).⁹

LGV is largely underdiagnosed in Europe partly because of the lack of appropriate LGV diagnostics, particularly in Eastern Europe and Central/Western Asia.⁷ According to ECDC data, in 2019, 3112 cases of LGV were reported in EU/EEA countries, almost all among men who have sex with men.³³ However, 87% of the reported cases were from only four countries (France, the Netherlands, Spain, and the UK), indicating variations in the occurrence or reporting of this infection across Europe. LGV is barely reported in non-EU/EEA countries, where only Belarus, Serbia, and Ukraine report LGV cases.⁹

Epidemiological data on STIs in the WHO European Region must be interpreted in the context of high heterogeneity in access to testing, testing policies, diagnostic techniques, surveillance systems, and reporting practice. Across country comparisons may be biased due to several factors: i) countries with higher testing rates and testing programmes for key populations will likely identify higher numbers as compared to countries with reduced access to testing, possibly due to stigma or discrimination,³⁴ ii) infections that are routinely screened for at the population level in some countries (e.g., CT in the UK, or PrEP-associated screening in western countries)^9,35,36 will result in a higher number of notifications; iii) countries with wider access to sensitive molecular testing will identify more infections.³⁷ iv) countries with more consolidated surveillance (i.e., adequately implemented in routine practice) and more complete reporting will show higher rates than countries with less efficient systems.38, 39, 40, 41 This highlights the unequal availability and robustness of epidemiology data and its strong correlation with underperforming surveillance systems and unequal access to STI testing, diagnosis and care across Europe.

Temporal trends

In the EU/EEA countries, overall notification rates per 100,000 population for all bacterial STIs reached an all-time high in 2019,⁴² but a modest decrease was observed in 2020 due to the impact of the COVID-19 pandemic followed by a rebound in 2021 (Fig. 2).⁴³ It is challenging to discern the extent to which behaviour changes or changes in testing performance/policy have contributed to the recent rise in STI notifications. Among countries that reported consistently to ECDC for the ten year period from 2012 to 2021 (Supplementary appendix Table S1), the notification rate of CT has slightly increased from 89 in 2012 to 96/100,000 in 2021 (Fig. 2, Supplementary appendix Table S2), partly attributed to intensified screening efforts and expanded availability of sensitive diagnostic techniques.^38,44 Notification rates of NG have shown a considerable increase from 6 in 2012 to 14/100,000 in 2021. A contributing factor is likely the enhanced screening (i.e., screening of asymptomatic patients and extragenital screening), particularly among men who have with men and especially those using pre-exposure prophylaxis for HIV (PrEP), resulting in a notable number of diagnoses among this population.⁴¹ Similarly, syphilis notifications have substantially increased from 5 in 2012 to 7/100,000 in 2021, with expansion of the population tested for syphilis and behaviour changes potentially playing a role.⁴⁰ Finally, the increase in the number of cases of LGV (from 1780 in 2015 to 3112 cases in 2019) may be primarily attributed to changes in testing policies among men who have sex with men in countries reporting a large number of cases, along with rising number of cases among those with negative HIV status.⁸

Fig. 2.

Trend of newly diagnosed infections over time (rate per 100,000) from 2012 to 2021 in the WHO European Region, including EU/EEA∗ and non-EU/EEA countries∗∗. Legend: ∗For the calculation of trends in EU/EEA countries, data were considered from those countries that reported consistently to ECDC for the ten-year period, namely 19 countries for CT, 23 for NG and 23 for syphilis (Supplementary appendix, Table S2 and S3). The UK ceased contributing data to the European Surveillance System in 2020 following UK withdrawal from the EU, thus is not included in the calculation of a trend. Details on number of CT, NG and syphilis cases reported every year by each country and the corresponding national notification rates are available from ECDC Surveillance Atlas.⁸ ∗∗For the calculation of trends in non-EU/EEA, data was extracted from an assessment conducted by the WHO/Europe in 11 countries,⁹ and national statistical reports from 6 countries. (Supplementary Appendix, Tables S3 and S4).13, 14, 15, 16, 17^,27

Availability of STI data in non-EU/EEA countries are more limited, with inconsistent collection of information on modes of transmission, hindering comparability with EU/EEA data and conclusions about STI epidemics. Data collated from national statistical reports indicates a steady decline in all three bacterial STIs in the past decade (Fig. 2 and Supplementary Appendix Tables S3 and S4).^9,13, 14, 15, 16, 17^,20, 21, 22, 23, 24, 25, 26, 27 In these countries, the notification rates of CT, NG, and syphilis have each declined per 100,000 population between 2012 and 2021, CT from 37.1 to 23.8, NG from 29.8 to 6.3, and syphilis from 24.4 to 11.3.

It is not clear if the steady decline in new diagnoses of all three bacterial STIs across the non-EU/EEA represents a true decline or reflects changing access to testing, changing testing practices or changing reporting practices. Therefore, notified cases may not reflect the true epidemiological situation and further in-depth exploration is needed. Despite the scarcity of evidence, several factors may have contributed to the observed decline, including reduced access to testing in some countries, possibly due to stigma or discrimination,³⁴ particularly affecting key populations (with limited data reported for this group), as well as differences in STI screening policies,^9,35 and access diagnostic methods.³⁷ Furthermore, implementation of PrEP for HIV programmes is considerably less extended in Eastern Europe and Central Asia compared to Western Europe countries, thus enhanced STI screening of men who have sex with men that use PrEP is likely more limited in the non-EU/EEA context.³⁶

Gender distribution, transmission category, age at diagnosis, and HIV coinfection in STI cases

Disaggregated demographic variables are available from the ECDC Surveillance Atlas for EU/EEA countries up to 2021, but they are more limited for non-EU/EEA countries, with the assessment published by WHO/Europe providing age and sex-disaggregated data available for CT and syphilis in 11 countries, and NG in 14 countries in 2019.⁹

In the EU/EEA among all countries that provided data to the ECDC in 2021 (Table 2, Supplementary appendix Table S5), 80% of CT notifications were among heterosexual females and males, with a slightly higher proportion among females (Table 2). NG notifications were five times higher in men compared to women, with 62% cases reported through homosexual transmission and 20% through heterosexual transmission in men. Syphilis diagnoses were predominantly reported among men, with 75% of cases attributed to homosexual transmission. Similarly, the non-EU/EEA countries reported a higher number of CT cases in women and more cases of syphilis and NG in men than in women.⁹ The male-to-female ratio of syphilis and NG cases in the non-EU/EEA countries were lower than in EU/EEA countries, 1.5 and 2.6, respectively. A small percentage of cases in the EU/EEA data were reported under “other” transmission category without further information about population groups such as non-heterosexual women, transgender and gender diverse individuals, highlighting the need for improved reporting of case characteristics at national and EU/EEA level.

Table 2.

Characteristic of bacterial STIs reported by EU/EEA countries to the European surveillance system, 2021.

Characteristics of cases in 2021a	Chlamydia	Gonorrhoea	Syphilis
No. of reported cases	184,542	46,723	25,270
No. of reporting countriesb	27	27	28
Male-to-female ratio	0.9:1	5:1	9:1
Range of national notification rates per 100,000 population	<1–627	2–123	<1 to 32
Distribution by age group (years)
0–14	0.3%	0.2%	0.1%
15–24	59.6%	27.4%	12.0%
25–34	26.7%	38.6%	31.9%
35–44	8.3%	20.4%	25.8%
45+	5.1%	13.4%	30.1%
Distribution by transmission category
No. of cases with info available	175,557	42,776	16,497
Heterosexual females	47.6%	16.6%	8.8%
Heterosexual males	31.5%	20.2%	15.2%
Men who have sex with men	19.4%	62.0%	75.4%
Otherc	1.4%	1.3%	0.6%
Median age in years by transmission category (IQR)
No. of cases with info available	64,259	24,241	13,393
Heterosexual females	21 (19–25)	23 (20–30)	31 (23–42)
Heterosexual males	23 (21–27)	27 (22–35)	37 (28–48)
Men who have sex with men	32 (26–42)	32 (26–40)	35 (28–45)
Percentage of cases with HIV-positive status by transmission category
No. of cases with info available	10,267	13,983	5279
Heterosexual females	3.1%	2.6%	1.7%
Heterosexual males	4.8%	5.1%	4.9%
Men who have sex with men	17.1%	14.3%	29.8%

^aIncludes cases from all countries that provided data for 2021 irrespective of consistency of reporting in the previous years and of surveillance system coverage.

^bDetails regarding reporting countries, reported cases, and notification rates by country can be found in the Supplementary Appendix Table S5.

^cThere is likely underreporting of STIs among non-heterosexual women, and transgender and gender diverse individuals, highlighting the need for better quality data at national an EU level.

Age-stratified analyses show higher proportion of most STIs among young age groups, although the contribution of these groups to the overall burden varies between countries and infections.^{9,32,38,40,41} In the EU/EEA dataset, the majority of CT cases (60%) were observed in people aged 15–24 years, in contrast only 12% of syphilis cases occurred in this age-group (Table 2). By transmission category, the median age for heterosexual individuals with CT/NG is younger (21–27 years), while for both homosexual and heterosexual men with syphilis, it is highest (35–37 years). Nonetheless, in 2022 and 2023, several Western European countries have noticed an increase of heterosexual transmission of NG among young people.45, 46

Completeness of information on HIV-serostatus varies in TESSy due to differences in surveillance data sources (e.g., clinical settings vs laboratory) and aggregated vs case-based reporting. Some publications indicate that people living with HIV are over-represented in bacterial STI cases,⁴⁷ likely due to more frequent screenings and increased contact with healthcare and contributing behaviour factors such as risk compensation or high-risk behaviour among men who have sex with men.⁴⁸

Sexually transmitted infections in key populations: multidimensional determinants and persistent inequities

As per the WHO, key populations—comprising men who have sex with men, transgender and gender diverse individuals, sex workers, people who inject drugs, and those in prisons and custodial settings—are defined as groups at increased risk for HIV, viral hepatitis, and STIs, emphasizing the need to prioritize prevention, diagnosis, and treatment.⁴⁹ Their increased risk stems from biological factors (e.g., enhanced anal intercourse efficiency, acute effects of STIs), individual behaviour (e.g., substance use, condomless sex), sexual networks (e.g., partner numbers, sexual venues), and structural factors (e.g., societal and health system discrimination, punitive laws, poverty).^50,51 STI prevention and control interventions are interconnected and need to address root causes, namely, the structural and social determinants of health.⁵²

• •Men who have sex with men

Globally, evidence of rising STI cases among men who have sex with men has been observed over the past decade. A systematic review in 2019 (58 studies) identified high prevalence of CT (6.9% [5.4–8.6]), NG (8% [6.3–11.0]), and early syphilis (5.3% [3–8]) among men who have sex with men tested before enrolment in PrEP,⁵³ and another review (34-point estimates) reported a syphilis prevalence of 3.4% [1.8–5.4].⁴⁰ In Europe, specifically, two extensive online surveys conducted among men who have sex with men across 46 European countries (EMIS-2010, EMIS-2017) observed a significant 76% increase in prevalence of self-reported CT/NG and an 83% increase in syphilis between 2010 and 2017.³⁹ The surveys identified factors associated with syphilis diagnosis, including living with HIV, having more frequent condomless anal intercourse with non-steady male partners, recent STI-screening, selling sex, and PrEP use.⁵⁴ The same data also suggested that changes in testing-practices and/or frequency likely played a larger role in reported increases in gonorrhoea and chlamydia than did changes in behaviour.

Men who have sex with men face some syndemic issues, including chemsex and other mental health issues, and could have external factors influencing their sexual behaviour, such as risk compensation during PrEP use, all of which could increase their susceptibility to STIs. Chemsex (i.e., use of certain types of drugs before or during sex) and ‘slamming’ (engaging in chemsex via injection) may also increase the risk of STIs transmission.⁵⁵ A recent study in Belgium developed and evaluated a mobile phone application (‘Budd-app’) for individuals engaging in chemsex, revealed behaviours such as prolonged sessions (mean of 17.5 h), polydrug use (95% of sessions), and unsafe dosing (49% of sessions).⁵⁶

Regarding the impact of PrEP for HIV on STI cases, studies yield conflicting results. Some suggest higher STI prevalence among PrEP users related to enhanced screening, biological synergies and risk compensation behaviours,57, 58, 59 while others find no significant increase in incidence or prevalence of STIs.^60,61 These results reinforce the interaction of preventive combination interventions and sexual behaviour and that PrEP programs should integrate high-quality STI and sexual health services.^53,62 (see the Series article on prevention).²

• •Transgender and gender diverse people

Transgender and gender diverse people encounter structural barriers, including criminalization, stigma, and discrimination like other key populations, but experience violence at very high rates.⁶³ They also face lack of legal recognition along with policy barriers to general and specialized healthcare,⁶⁴ making them much more susceptible to various health issues like substance use disorders, depression, and self-harm, HIV, and bacterial STIs. A systematic review (25 studies, 11 countries) reported varying prevalence of HIV in transgender women (0%–49.6%) and transgender men (0%–8.3%) in different studies, as well as varying prevalence of CT (2.7%–24.7%), NG (2.1%–19.1%), and syphilis (1.4%–50.4%) in transgender women, with limited data on transgender men.⁶⁵

• •Sex workers

Sex workers, including female, male, transgender, and gender diverse individuals, are highly susceptible to HIV and STI acquisition due to unsafe working conditions, sub-optimal access to health and prevention services, and barriers to consistent condom use.⁶⁶ Social determinants such as the lack of social protection, housing, food insecurity, reduced education opportunities, and disability further compound their risks.

A study in Barcelona revealed high prevalence of HIV (25%) and bacterial STI (CT 10%, NG 19%) among cisgender men and transgender women sex workers.⁶⁷ Syndemic conditions, alcohol, polydrug use, violence (affecting 40% of individuals), and lack of healthcare access were common impacting 79% of cisgender men and 68% of transgender women. Condomless anal sex (aOR 1.81) and frequent alcohol consumption (aOR 2.73) were associated with violence, and alcohol consumption (aOR 1.88) with having more clients.⁶⁸ Another study in the US found a 40% HIV prevalence and high STI prevalence (CT 18%, NG 10%), with insertive anal sex with clients increasing the risk (RR 3.48).⁶⁹

Solutions to address high STI prevalence among sex workers include improving sexual health, addressing violence, and considering sex work legislation.^70,71 An ecological analysis (27 European countries) showed that legalising or decriminalizing aspects of sex work (n = 17 countries) is associated with lower HIV prevalence among sex workers compared to countries criminalising sex work (n = 10 countries).⁷² Multicomponent and peer-co-designed approaches have also proven effective in improving mental and physical health outcomes for female and street-based workers, with limited data for other sex workers.⁷³

• •People who inject drugs and people in prisons

People who inject drugs are a recognized key target population for HIV and viral hepatitis prevention and to a lesser extent for bacterial STIs. Although the prevalence of STIs observed is low, scant condom use, higher risk to be victims of sexual violence and to exchanging sex for money, makes it necessary for prevention programs to include messages related to sexual risk practices, especially among young people and women.⁷⁴ On the other hand, the prevalence of bacterial STIs among people in prisons is heterogeneous in selected studies (CT 1.0%–6.7%, NG 0.6%–7.8%), with recreational drug use, low educational levels, and sex without a condom as major risk factors.⁷⁵

• •Migrants and refugees

Migrants and refugees are not classified as key population for STIs by WHO, yet structural inequities like sexism, racism, heterosexism, and poverty contribute to elevated STI cases within this group.⁵² A study conducted in Milan involving 537 undocumented migrant women from 39 countries found high STI prevalence (HPV 24%, CT 8%).⁷⁶ In Malta, a study of 143 migrants from outside Europe attending a sexual health clinic revealed a 73% STI prevalence,⁷⁷ along with recorded experiences of sexual violence leading to sexual health complications. Recognizing these risks during migration is essential for comprehensive STI prevention and care.

A scoping review (11 articles, 5 focused solely on Europe as the receiving region, and 6 were mixed but included Europe) identified six upstream social and structural determinants that undermine the sexual and reproductive health and rights of migrants and refugees in high income countries. These determinants are the economic crisis in Europe, legal entitlements barriers, inadequate resources for migrant health, poor living and working conditions (e.g., living in underserved areas, detention or reception centres, confiscation of passports, etc), cultural and linguistic barriers, and stigma/discrimination.⁷⁸ Another review including experiences worldwide identified similar determinants.⁷⁹ This highlights the need for targeted policy-making and planning to address structural challenges, and the importance of improving STI/HIV awareness, testing, and early service utilization among migrants.

New challenges beyond known determinants for STIs

• •Social and sexual networks

Social and sexual networks play an important role in STI transmission,⁵² and network epidemiology has advanced our understanding of “risk-potential networks” driving STI spread.⁸⁰ Factors like the number of connections, being part of a core group, how different groups mix, the relationship between individuals, and having multiple concurrent sexual partners⁸¹ influence behaviours and infectious outcomes. For example, core groups with both a high STI prevalence and a high number of multiple inter-connected partners are believed to be particularly relevant for explaining increases of syphilis.^54,82 Conversely these core groups are thought to be much less important in the overall increase in cases of CT and NG.⁸³ Moreover, the role of networks in the transmission of Monkeypox virus (MPXV) during the early phase of the outbreak in Belgium has been recently described.⁸⁴

• •Pandemics: COVID-19

Several studies have documented a decrease of STI consultations and/or diagnoses during the first COVID-19 wave in 2020, particularly with regards to asymptomatic infections. However, it remains challenging to ascertain whether these reductions are real or due to underdiagnoses and underreporting during lockdown periods.⁴³

An impact assessment survey (EuroTEST COVID-19) collected 98 responses from secondary care clinics, community sites, and NGOs (in 23 EU/EEA and 11 non-EU/EEA countries). The survey compared 2020 data to 2019 and found 95% of respondents experienced testing volume decreases, with 64% facing severe disruptions to testing provisions.⁸⁵ Volumes of community-based voluntary counselling and testing (CBVCT) dropped significantly (>50%) due to lockdown closures (69%), reduced attendance in sites that continued operating (66.2%), and limited staff (59.7%).⁸⁵ No differences were observed between EU/EEA and non-EU/EEA countries.

The COVID-19 pandemic impacted sexual and reproductive health (SRH) services leading to unmet needs among the general population. Data from a UK study among 6654 participants aged 18–59 years found that 20.8% used SRH services, and 9.7% reported attempting but being unable to access the service they needed.⁸⁶ In addition to disruptions in access to SRH services, members of populations in vulnerable situations were at increased risk of indirect impacts arising from responses to COVID-19, such as isolation, loss of income and residential instability.^87,88

The challenges encountered throughout all stages of the STI care cascade during epidemics like COVID-19 should serve as lessons, informing the development of effective interventions to mitigating their impact and enhance future public health preparedness.

Molecular epidemiology: networks of transmission and clustering

Molecular tools are essential for understanding STI epidemiology, especially for NG, allowing the investigation of transmission networks and antimicrobial resistance (AMR) monitoring. Whole genome sequencing (WGS) enhances resolution in describing NG transmission networks compared to multi-locus sequence typing (MLST), yet the latter is more cost-effective and feasible with lower-volume DNA samples. On the other hand, sequencing of CT and T. pallidum has been limited because these are difficult-to-culture organisms. Nevertheless, recent developments in DNA enrichment techniques have enabled advances in the application of WGS to these bacteria.⁸⁹

Tracking networks of transmission

Sequencing of NG isolates indicates a high molecular concordance among sexual contact pairs,⁹⁰ with most genomically linked cases diagnosed within a short time (over half within 30 days, and three-quarters within 90 days).⁹¹ Moreover, NG cases are predominantly acquired locally, with a smaller proportion linked to distantly situated national or international transmission events. In a study in Brighton (2011–2015)⁹¹ almost three-quarters of cases were local, with a smaller proportion linked to other national (18%) or international (9%) populations. Travel-related partnerships involved a minority of unique sequence types acquired outside the area.⁹⁰ The impact of COVID-19 lockdown was also assessed by WGS in the Netherlands, showing a marked increase in a single specific NG strain with a reduction in genetic diversity consistent with extensive local transmission but limited transnational transmission.⁹² Regarding NG networks of transmission in Europe, most studies have identified a difference in sequence types circulating between homosexual men and heterosexual individuals.⁹³ A few clusters involve solely homosexual and heterosexual men, but not heterosexual women, suggestive of mixing between gay and heterosexual-identifying men who have sex with men. Sub-groups with sequence types that vary based on ethnicity, age, particular sexual behaviours or HIV status have all been described^90,93,94 indicating distinct sexual networks.

In the case of syphilis, T. pallidum slow acquisition of single nucleotide polymorphisms hampers molecular transmission studies. Nevertheless, a UK study combining genomic data and meta-data demonstrated distinct homosexual and heterosexual transmission networks with little bridging between the two populations.⁹⁵ Further evidence revealed the presence of two concurrent major lineages (SS14 and Nichols) circulating in Europe and globally,⁹⁶ with most major sub-lineages detected across Europe consistent with international sexual network connections. Additionally, genotypic resistance to azithromycin has been shown to have emerged multiple times in different locations,⁸⁹ suggesting no single newly emergent strain drives the recent macrolide-resistant syphilis epidemic in Europe.

Molecular epidemiology for CT has prominently involved monitoring diagnostic-escape variants, exemplified by the nvCT variant, initially identified in Sweden in 2006 with a plasmid DNA 77bp deletion. This deletion made a specific nucleic acid amplification test (NAAT) ineffective, leading to undiagnosed infections and subsequent nationwide dissemination of the variant.⁹⁷ Modifications in testing protocols subsequently reduced infections caused by this strain from over 56% in 2007 to 6.5% in 2015.⁹⁷ In February 2019, a diagnostic-escape CT mutant (23S rRNA, C1515T) emerged in Finland (10% in 2019, 2% in 2022) and spread to Norway (84% prevalence in 2020), while another escape mutant (23S rRNA, G1523A) became prevalent in Denmark (95% in 2020).⁹⁸ Moreover, specific LGV CT strains, have been identified as the cause of recent epidemics among men who have sex with men, as demonstrated through genotyping of the outer-membrane-protein (OmpA).⁹⁹ An LGV epidemic was caused by the L2b variant in 2004 in Europe, and new variants have been identified later, but all derive from the initial L2b strain.

NG antimicrobial resistance

Sequencing to identify drug-resistant NG can provide insights into the feasibility of resistance-guided treatment. If most resistant cases are linked to one or a few mutations, standard NAATs used in clinical practice could be adapted for detection, and allow diversification of treatment for NG¹⁰⁰ (see the Series article on treatment).⁴ For instance, most quinolone and cefixime resistance results from few mutations,¹⁰¹ allowing genotypic diagnosis of resistance, faster than phenotypic methods. Notably, a test targeting the gyrA mutation which causes >90% of cases of quinolone resistance holds potential for use at individual level.¹⁰⁰

Moreover, the integration of antimicrobial resistance surveillance for NG into existing monitoring platforms, like the European Gonococcal Antimicrobial Surveillance Programme (Euro-GASP), has proven advantageous. Euro-GASP provides a large-scale, representative sample of NG isolates in Europe,¹⁰² enabling the repeated assessment of this dataset for tracking of specific lineages. Notably, the NG-MAST G1407 lineage includes all cephalosporin-resistant isolates (5% cefixime, 1% ceftriaxone), despite it also includes a significant number of susceptible isolates.¹⁰³ This lineage was highly prevalent from 2009 to 2013, representing 23% of isolates in 2009–10, and 17% of isolates in 2013. However, its prevalence decreased over time, shifting from predominantly affecting homosexual to heterosexual men. By 2018, the transmission of this lineage had declined to 2.1%. The overall decline in this strain has resulted in a higher proportion of circulating strains that are now susceptible to cephalosporins.¹⁰⁴ Moreover, the 2018 EURO-GASP identified ciprofloxacin resistance remained high (>50%) nearly all (>99%) due to S91F mutation in gyrA, and a novel genogroup (NG-MAST G12302) with low-level azithromycin resistance (8%) emerging predominantly in pharyngeal isolates of men who have sex with men.¹⁰⁵ Overall resistance patterns varied across countries (25%–75%); with azithromycin resistance from 0% to >50%, and cefixime resistance from <1% to 25%. Euro-GASP data showed similar levels of AMR between European-born and non-European-born individuals.¹⁰⁶

Emerging STIs: zoonotic origins and shifts in transmission routes

Emerging STIs in Europe are characterized by an increasing number of infections that were previously uncommon or that are newly recognized, along with certain STIs that were once limited to other geographic regions becoming more common within specific European populations. These infections can be attributed to diverse factors such as zoonotic cross-species transmission (e.g., mpox), evolving routes of transmission resulting from changes in sexual behaviour (e.g., Hepatitis A, B, C, D or Shigella), global travel and migrations (e.g., Zika and Ebola), or geographical expansion to a new continent (e.g., LGV C. trachomatis) (Table 3).

Table 3.

Emergent Sexually Transmissible Pathogens in Europe.

Pathogen	Traditional Transmission Mechanism	Sexual Transmission Mechanisms	Groups at increased risk of sexual acquisition	Frequency
Mpox	Zoonotic transmission	Direct contact with lesions, body fluids	Men who have sex with men	2022 pandemic
Hepatitis A	Faeco-oral transmission	Oral-anal sex	Men who have sex with men	Periodic outbreaks
Hepatitis B	Percutaneous of permucosal contact with body fluids	Unprotected sex in unvaccinated individuals	Sexually active heterosexual individuals and men who have sex with men	Ongoing transmission
Hepatitis C	Specific sexual practices (i.e., fisting), injecting drug use, contaminated blood products	High-risk anal sex involving contact with blood	Men who have sex with men	Ongoing transmission
Hepatitis D	Injecting drug use, contaminated blood products	High-risk anal sex involving contact with blood. Super-infection in chronic hepatitis B or simultaneous co-infection with hepatitis B	Men who have sex with men and sex-workers	Ongoing transmission
Shigella	Faeco-oral transmission	Oral-anal sex	Men who have sex with men	Ongoing transmission
Zika	Aedes mosquito bite	Transmission via genital fluids	Partners of individuals contracting Zika, including travel-related	Sporadic case reports
Ebola	Contact with infected body fluids	Transmission via genital fluids	Partners of individuals contracting Ebola	Sporadic case reports
LGV	Sexual contact with infected individuals in the tropics	Sexual contact in temperate climates	Men who have sex with men, core groups	Ongoing

Zoonotic STI origins, cross-species transmission, and human adaptation

Several STIs have a zoonotic origin or are presumed to have originated from animals, with HIV being a prominent example of zoonotic STI that has transitioned to human-to-human transmission. Some emerging STIs, like mpox, are still classified as zoonoses, and there is a long list of pathogens with close related microbial species in other animals, including Haemophilus ducreyi,¹⁰⁷ T. pallidum subsp. pertenue¹⁰⁸ and others.

Determining zoonotic origins is complex due to confounding factors.¹⁰⁹ HIV is a well described zoonotic STI, with its zoonotic source in chimpanzees discovered in 1999,¹¹⁰ but it was not immediately apparent when it was first reported in 1981.^111,112 Zoonotic events involve host susceptibility, whilst the likelihood of these resulting in an epidemic is impacted by the ability to sustain human-to-human transmission,^110,113 and mutation and recombination may play roles in overcoming barriers to cross-species transmission and sustaining transmission.¹¹⁴

Mpox is an STI with a zoonotic origin that caused a global epidemic in 2022, primarily affecting men who have sex with men.¹¹⁵ Human cases were first reported in the Democratic Republic of the Congo in 1970 with sporadic subsequent outbreaks in Africa linked to zoonotic transmission events from rodents.¹¹⁶ The outbreak in 2022 involved direct contact with lesions during sex as the major mode of transmission,^117,118 and caused severe disease in people with advanced HIV.¹¹⁹ Genetic sequencing of 2022 revealed MPXV lineage 2b strains that had been circulating in Nigeria in 2017 with notable divergence due to APOBEC-related mutations.¹²⁰ This indicates significant human-to-human transmission and possible host adaptation. A study of 1900 MPXV genomes found thirteen derived lineages with ten distinct non-synonymous mutations in genes related to immune evasion, virulence and host recognition that likely contribute to the rapid evolution and diversification of current MPXV lineages.¹²¹ Mpox growth was linked to sexual networks with many partners,¹²² with behaviour change and vaccination being potential control measures. This highlights the role of core groups in sustaining transmission.¹²²

Sex as emerging route of transmission

Several pathogens, like Hepatitis A virus, Shigella, formerly transmitted primarily through the enteric route, have seen ongoing transmission among men who have sex with men due to sexual transmission mechanisms like oral-anal sex. In high-income countries Shigella spp, an enteric pathogen causing acute colitis, has been primarily acquired through travel. However, since the 1970s, cases of sexually transmitted shigella have emerged particularly among men who have sex with men. Certain lineages of both Shigella flexneri and Shigella sonnei,¹²³ primarily transmitted within this population, have acquired antibiotic resistance to multiple antibiotics and spread rapidly through sexual networks spanning countries and continents.¹²⁴ The emergence of Shigella in this population is linked to specific sexual practices such as rimming (oral-anal sex).¹²⁵ In the UK sexual transmission amongst men who have sex with men now accounts for nearly 50% of reported cases. Multiple waves of transmission have occurred driven by strains with differing antibiotic resistance patterns. Initially, the most notable pattern was azithromycin resistance, associated with serotype 3A S. flexneri, but subsequent S. sonnei strains resistant to ciprofloxacin have also emerged and more recently plasmid-mediated extended beta-lactam resistance has been described, initially in S. sonnei and with emerging evidence of horizontal gene transfer to S. flexneri strains.^126,127

Similarly, sustained sexual transmission of Hepatitis A virus among men who have sex with men is well-documented, leading to recommendations for vaccination in this population. However, the implementation of vaccination strategies has been inconsistent, leading to periodic outbreaks like a multi-country outbreak in Europe in 2016–17, with approximately 4100 confirmed cases.¹²⁸ Subsequent outbreaks have been reported in Hungary, Croatia, and Romania in 2022–2023,¹²⁹ as men who have sex with men in these and other Eastern Europe and Central Asia countries remain unvaccinated against hepatitis A.

In addition, other pathogens like Hepatitis C virus, formerly transmitted by percutaneous transmission, have also demonstrated instances of sexual transmission through contact with blood during sex. The risk is higher among men who have sex with men,¹³⁰ particularly among those who also are living with HIV or those who are on PrEP for HIV, because of differences in sexual practice that incur a greater risk of trauma and blood exposure.¹³¹ Transmission risk is associated to condomless anal sex, traumatic sexual practices (such as fisting, group sex and chem-sex), injecting drugs, and recent diagnosis of an ulcerative STI.^131,132 More recently widespread access to directly acting anti-viral agents which are highly effective in the treatment of hepatitis C has significantly reduced the burden of primary Hepatitis C virus infections in Europe with the majority of cases now due to re-infections amongst men who have sex with men. Similarly, the incidence of acute Hepatitis B virus infection in Europe has declined due to vaccination programmes. Traditionally, Hepatitis B virus has been transmitted through various body fluids, and among acute cases nowadays, acquisition is most commonly reported among both heterosexual individuals and men who have sex with men who are unvaccinated.¹³³

Geographic extension of STIs

LGV is an example of geographical expansion, which was previously limited to heterosexual individuals in East and West Africa and has since 2003 expanded among networks of men who have sex with men in high-income countries due to high-risk sexual behaviours, leading to an endemic level of transmission.⁶ The reasons for the expansion of LGV cases from tropical to temperate regions are not fully understood. Independent risk factors for LGV in Europe include concurrent genital ulcerative disease, HIV, previous STIs, unprotected anal sex, recent travel, and meeting partners online. The rectal-urethral LGV ratio is 15:1, suggesting that brief urethral infections are sufficient for transmission but too short to manifest symptoms, in addition to an oro-anal transmission route.¹³⁴ This implies that sustaining transmission requires frequent sexual encounters with multiple partners.

Zika virus and Ebola virus have also been classified as emerging STIs because of transmission via genital fluids. Several Ebola cases during the late phase of the 2014 to 2016 epidemic in West Africa were attributed to sexual transmission,^135,136 with viral RNA detected in semen for an average of 19 months, but no cases have been reported in Europe. In 2015, Zika virus caused a large epidemic in the Americas, with imported cases in Europe, including instances of sexual transmission^137,138 (infectivity in semen up to 30 days), but current cases are low (22 in 2020).

Conclusions

Looking forward, this report highlights several important aspects that require attention and action. First, given the important regional variations in reported STI notification rates across the WHO European Region, efforts should be made to improve and harmonize epidemiological surveillance systems, as well as monitoring and evaluation protocols of programs and services, including access to testing and screening in instances where public health benefit has been demonstrated. Regarding surveillance, disaggregation for men who have sex with men at high risk, non-heterosexual women and transgender and gender diverse individuals should be encouraged. Second, to address the community-transmission of STIs, especially among vulnerable populations, it is important to recognize the role played by dense sexual networks and a combination of social and structural factors. New data collection approaches and research methods should be put in place to describe these factors, measure how they contribute to STI-burden and integrate them in prevention strategies and programmes. Enhancing case-based reporting with programmatic, social, behavioural, and structural data on STI risk factors and determinants is crucial to better understand the dynamics of these epidemics to improve the design and effectiveness of multilevel preventive interventions. Finally, close monitoring and research are needed to identify of potential new emerging STIs, new antimicrobial resistance patterns, and identify and strengthen effective preparedness measures.

Contributors

VP and OMi conceived the review and coordinated its preparation. MAR and OMa conducted the search of surveillance data. OMa, OMi, and JC wrote the part on epidemiological overview. MM and OMi wrote the part on molecular epidemiology. CF, VP, IR, and KB wrote the part on key populations. MM, AT, AC and OMi wrote the part on emerging STIs. All authors reviewed and edited the final version and approved the final manuscript.

Editor note

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Declaration of interests

The authors declare no competing interests.