Evaluating syphilis surveillance in Finland 2020-2024: identifying missed reinfections

Abstract Background The Finnish Institute for Health and Welfare (THL) received feedback that the current syphilis surveillance system may not adequately capture reinfections. This is due to a 50-year combination period defined in the Communicable Diseases Decree (146/2017, §2), which aggregates all notifications from the same individual into one case. As a result, repeated infections in high-risk groups may be missed, while cured infections (serological scars) are included despite limited public health relevance. We analysed surveillance data to assess these limitations and propose improvements. Methods We analysed syphilis data reported to the National Infectious Diseases Register (NIDR) during 2020-2024, including both aggregated cases and all clinician notifications, even those excluded by the combination logic. We also consulted Finland's largest clinical microbiology laboratory. Results NIDR recorded 1,541 cases in 1,541 individuals. However, 2,156 clinician notifications were submitted, meaning 615 (29%) were aggregated with existing cases, suggesting likely reinfections. Among cases, 11% were marked as serological scars, but this field was missing in 15% of records, indicating underestimation. Stakeholders from Finland's largest microbiology laboratory acknowledged these issues, supported the changes proposed by THL, and agreed to continue collaboration to improve surveillance. Conclusions Reinfections are not visible in NIDR due to the 50-year combination period, limiting accurate trend detection and risk group monitoring. Although the inclusion of non-infectious cases is acceptable, improving the completeness of scar classification is essential. We recommend shortening the combination period and strengthening scar data reporting. Key messages • Finland’s syphilis surveillance underestimates reinfections and often lacks complete scar data. • Shortening the combination period and improving scar data would better reflect trends and support public health action.