Incidence of medically attended Lyme borreliosis across endemic regions of six European countries (BOLD): a prospective multicentre active surveillance study

Background Lyme borreliosis (LB) is endemic in many European countries. Comparing LB incidence between countries is challenging because case definitions and surveillance methodologies are inconsistently implemented. The prospective, epidemiological Burden of Lyme Disease (BOLD) study used active surveillance to characterise LB burden in Europe. Methods LB patients were evaluated at 14 healthcare practices in high LB incidence areas in six European countries based on EUCALB-derived case definitions using standard of care (SOC) laboratory diagnostics (April 2021–December 2022). Enrolled cases were interviewed for medical/care histories and underwent additional standardised laboratory testing. OspA types were determined by in-silico sequence analysis where possible. Findings Among participating sites, 797 LB cases were identified using SOC diagnostics. The incidence (per 100,000 population per year) of LB was 657 for 2021 and 661 for 2022. Incidence was highest in the Slovakian sites (2021, 3059; 2022, 1860) and lowest in the Polish (2021, 209) and German sites (2022, 75). When standardised laboratory testing was considered for disseminated cases, incidences remained relatively consistent for 5 of the 6 countries. Borrelia afzelii (serotype 2) was identified in 30/33 (91%) isolates. Interpretation LB incidence was high in this prospective epidemiological study spanning multiple countries. The heterogeneity in LB incidence across regions illustrates inherent difficulties in aggregating estimates and highlights a need for robust regional monitoring and reporting, and standardised methods. Public health interventions for the prevention of LB should be considered. Funding This analysis was supported and jointly funded by Valneva and Pfizer Inc. as part of their co-development of a Lyme Disease vaccine.