Syphilis epidemiology in Finland, 2020–2025: influence of surveillance case definitions on the characterisation of trends and affected populations
Abstract
Background: Finland has reported a lower male-to-female ratio for syphilis than most European Union/European Economic Area (EU/EEA) countries and experienced a steep increase in cases after 2021, driven mainly by an increase in reported cases among women. We assessed how Finnish surveillance case-generation rules affect syphilis surveillance outputs and their comparability with those from other EU/EEA countries.
Methods: Finland operates a nationwide passive surveillance system in which laboratory and clinician notifications of active, past (serological scar), and uncertain syphilis infections are mandatory. Under the current case-generation logic, any of these notifications can initiate a case and notifications concerning the same individual are combined into a single case over 50 years. Consequently, reinfections during this period are not captured as separate cases. We analysed national surveillance data from 2020 to 2025, evaluating completeness and timeliness, classifying cases by the infection status of their initiating signal, examining trends by citizenship and sex, and assessing obscured reinfections. We compared the current case-generation logic with an alternative logic using a one-year aggregation period and allowing only clinician notifications indicating active infection to generate cases. National surveys assessed notification practices and system acceptability among clinicians and clinical microbiology laboratories.
Results: Core surveillance variables had high completeness and reporting was timely. The post-2021 increase among women was largely driven by Ukrainian nationals, among whom 73% of cases were women and only 16% were initiated by notifications indicating active infection; most were initiated by notifications compatible with uncertain or past infection. Notifications compatible with reinfection that were merged with existing cases under the aggregation rule occurred predominantly among men who have sex with men. Compared with the current logic, the alternative logic generated fewer cases from the same notifications, particularly among women, which increased the male-to-female ratio, bringing Finnish epidemiological patterns closer to those reported in other EU/EEA countries. Surveys completed by 37/152 clinicians and representatives of 8/9 laboratories identified variation in notifying past and uncertain infections, although notification was generally considered easy.
Conclusions: Surveillance-system design can substantially influence reported disease trends and the apparent populations affected, particularly when laboratory evidence persists after treatment or population movements alter screening patterns. Generating reported cases from notifications indicating active infection and shortening the combination period could improve the interpretability and public health usefulness of Finnish syphilis surveillance system.
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BibTeX
@article{czarnecki2026syphilis,
title = {{Syphilis epidemiology in Finland, 2020–2025: influence of surveillance case definitions on the characterisation of trends and affected populations}},
author = {Jakub Czarnecki and Lotta Siira and Maria Francesca Manca and Tuula Hannila-Handelberg and Teemu Möttönen and Jukka Ollgren and Jan-Erik Löflund and Satu Kurkela and Hanna Jarva and Eija Hiltunen-Back and Kirsi Liitsola},
journal = {BMC Public Health},
year = {2026},
month = nov,
doi = {10.1186/s12889-026-30360-6},
url = {https://link.springer.com/article/10.1186/s12889-026-30360-6},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Syphilis epidemiology in Finland, 2020–2025: influence of surveillance case definitions on the characterisation of trends and affected populations
AU - Jakub Czarnecki
AU - Lotta Siira
AU - Maria Francesca Manca
AU - Tuula Hannila-Handelberg
AU - Teemu Möttönen
AU - Jukka Ollgren
AU - Jan-Erik Löflund
AU - Satu Kurkela
AU - Hanna Jarva
AU - Eija Hiltunen-Back
AU - Kirsi Liitsola
PY - 2026
DA - 2026/11/13
JO - BMC Public Health
DO - 10.1186/s12889-026-30360-6
UR - https://link.springer.com/article/10.1186/s12889-026-30360-6
AB - Background: Finland has reported a lower male-to-female ratio for syphilis than most European Union/European Economic Area (EU/EEA) countries and experienced a steep increase in cases after 2021, driven mainly by an increase in reported cases among women. We assessed how Finnish surveillance case-generation rules affect syphilis surveillance outputs and their comparability with those from other EU/EEA countries. Methods: Finland operates a nationwide passive surveillance system in which laboratory and clinician notifications of active, past (serological scar), and uncertain syphilis infections are mandatory. Under the current case-generation logic, any of these notifications can initiate a case and notifications concerning the same individual are combined into a single case over 50 years. Consequently, reinfections during this period are not captured as separate cases. We analysed national surveillance data from 2020 to 2025, evaluating completeness and timeliness, classifying cases by the infection status of their initiating signal, examining trends by citizenship and sex, and assessing obscured reinfections. We compared the current case-generation logic with an alternative logic using a one-year aggregation period and allowing only clinician notifications indicating active infection to generate cases. National surveys assessed notification practices and system acceptability among clinicians and clinical microbiology laboratories. Results: Core surveillance variables had high completeness and reporting was timely. The post-2021 increase among women was largely driven by Ukrainian nationals, among whom 73% of cases were women and only 16% were initiated by notifications indicating active infection; most were initiated by notifications compatible with uncertain or past infection. Notifications compatible with reinfection that were merged with existing cases under the aggregation rule occurred predominantly among men who have sex with men. Compared with the current logic, the alternative logic generated fewer cases from the same notifications, particularly among women, which increased the male-to-female ratio, bringing Finnish epidemiological patterns closer to those reported in other EU/EEA countries. Surveys completed by 37/152 clinicians and representatives of 8/9 laboratories identified variation in notifying past and uncertain infections, although notification was generally considered easy. Conclusions: Surveillance-system design can substantially influence reported disease trends and the apparent populations affected, particularly when laboratory evidence persists after treatment or population movements alter screening patterns. Generating reported cases from notifications indicating active infection and shortening the combination period could improve the interpretability and public health usefulness of Finnish syphilis surveillance system.
SN - 1471-2458
ER -