The study was carried out in the Czech Republic, Germany, Poland, Slovakia, Slovenia and Sweden, regions deliberately chosen for their high historical burden of disease. Each participating site had recorded a historical Lyme borreliosis incidence of at least 500 per 100,000 population per year. Active surveillance began when each site activated between April and July 2021 and ran for twelve months, with case identification continuing through the end of 2022. Researchers tracked patients through routine primary care contacts and medical records, and sites also reached out to local hospitals and emergency departments to minimise missed cases.

Of the 994 suspected Lyme borreliosis cases identified during the study period, 797, or 80.2 percent, fulfilled the study’s case definition, which was developed from the European Union Concerted Action on Lyme Borreliosis criteria. The cases comprised 94 from the Czech Republic, 63 from Germany, 30 from Poland, 173 from Slovakia, 95 from Slovenia and 342 from Sweden. Most patients, 77.2 percent, were diagnosed with erythema migrans, the characteristic expanding skin rash of early infection, while 18.4 percent had Lyme arthritis, 0.5 percent Lyme neuroborreliosis and 0.3 percent Lyme carditis. The mean age of cases was 53.2 years, and 54.7 percent were female.

The overall incidence of medically attended Lyme borreliosis at the study sites, excluding one Polish site where patients with tick bites were routinely given preventive antibiotics, was 657 per 100,000 population in 2021 and 661 per 100,000 in 2022. Slovakia stood out dramatically. Its two sites recorded 3,059 cases per 100,000 population in 2021 and 1,860 per 100,000 in 2022, with one Slovakian site reaching 4,366 per 100,000 in the first year. The Swedish site had the second-highest incidence, at 1,541 per 100,000 in 2021 and 1,737 per 100,000 in 2022. Poland recorded the lowest figure in 2021 at 209 per 100,000, while the German sites were lowest in 2022 at 75 per 100,000.

That variation was not only between countries but also within them. German sites recorded incidences ranging from 166.9 to 456.4 per 100,000 in 2021, and year-to-year changes were inconsistent: one Czech site saw its incidence double between 2021 and 2022, while the German sites overall dropped to about a third of their previous level. The authors attribute this spatial and temporal variability to the ecology of the tick vector Ixodes ricinus, whose abundance fluctuates with climate, rodent populations and other environmental factors, as well as to differences in healthcare practices. They caution that high-incidence sites were deliberately selected, so the results cannot be extrapolated to entire countries, particularly where only one site participated.

A central strength of the study was its standardised laboratory component. Of the 797 cases, 362 individuals with suspected Lyme disease enrolled in the study, and 315 were confirmed as medically attended cases. These participants underwent a modified two-tier serological test as well as direct detection methods, including polymerase chain reaction and bacterial culture of skin biopsies, with samples analysed at central laboratories. Overall, 198 of the 359 enrolled cases tested, or 55 percent, were positive by at least one test. Serology was positive in 53.1 percent of those tested, PCR in 37.2 percent of skin biopsies, and culture in only 7.3 percent, a low recovery rate the authors partly attribute to antibiotic use before biopsy in 41.5 percent of cultured samples.

When the researchers calculated a study laboratory confirmed incidence, it fell to 414.2 per 100,000, compared with 663.6 per 100,000 for medically attended cases. They also computed a combined incidence, applying clinical diagnosis for erythema migrans, which is typically diagnosed without laboratory testing in routine practice, and standardised laboratory confirmation for disseminated cases. That combined figure was 603.8 per 100,000. For every country except Slovakia, the combined incidence was at least 90 percent of the medically attended incidence, indicating broad consistency between standard-of-care and study laboratory testing for disseminated disease. Slovakia’s combined incidence remained the highest of any country at 1,935 per 100,000, suggesting that differences in local laboratory practice alone cannot explain its exceptionally high rates.

The study also yielded molecular epidemiology data. Among 86 erythema migrans cases with skin biopsies tested by culture or PCR, 34 isolates were positive and could be sequenced to determine genospecies and serotype. Borrelia afzelii, corresponding to serotype 2, dominated, accounting for 30 of 33 typable isolates, or 90.9 percent, from participants in Sweden, Slovenia, the Czech Republic, Germany and Poland. Borrelia garinii serotype 6 appeared in two cases and serotype 8 in one. The authors note that the scarcity of non-afzelii isolates, even in a multi-country prospective study, highlights the challenge facing any serotype-specific evaluation, such as the efficacy of a serotype-targeted vaccine.

Comparisons with public health surveillance data revealed consistent under-ascertainment. At every participating site, the incidence of Lyme borreliosis measured by active surveillance exceeded the incidence reported through routine surveillance, though the size of the gap varied substantially between regions and countries. This aligns with earlier estimates, including a study cited by the authors suggesting that 2.7 times more annual cases occurred in Norway, Finland and Poland than surveillance systems captured. The researchers attribute past difficulties in comparing national figures to inconsistent implementation of case definitions, differing laboratory testing practices, and reliance on passive surveillance or retrospective designs, all of which the BOLD methodology was designed to overcome.

The study had limitations the authors are careful to spell out. Sites were located in high-endemic areas and cannot represent national averages. Only two years of data were captured, and tick-borne disease rates can vary considerably year to year. Pre-season baseline blood samples were unavailable, so serology-positive results could not be distinguished from antibodies generated by infections before the study began. No synovial fluid specimens were obtained from suspected arthritis cases, erythema migrans diagnoses without laboratory confirmation carry a risk of misdiagnosis, and asymptomatic infections were not captured. The study also began as the COVID-19 pandemic was waning, and altered healthcare-seeking behaviour, including closed practices in Slovakia shortly before the study started, may have affected both case capture and comparisons with surveillance data from the same period.

Funding and authorship deserve mention: the analysis was jointly supported by Valneva and Pfizer as part of their co-development of a Lyme disease vaccine candidate, and Pfizer was the regulatory sponsor involved in study design, data analysis and the decision to submit the paper. Several authors are Pfizer employees. The study was also designed in part to identify potential trial sites for an outer surface protein A vaccine candidate, underscoring the commercial context of the burden data. Even so, the methodological rigour of the active surveillance design gives the incidence estimates unusual weight. The authors conclude that the figures likely reflect true differences in Lyme disease risk across endemic regions of Europe rather than artefacts of surveillance methodology, and they argue that the marked heterogeneity between sites shows why aggregated national estimates obscure local risk. High-incidence foci, they write, exist at many sites across the continent, and public health interventions for prevention, including vaccination, should be considered.

Subject of Research: Prospective active surveillance of medically attended Lyme borreliosis incidence in endemic regions of six European countries

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

Article References: Halsby, K., Loew-Baselli, A., Moniuszko-Malinowska, A., Strle, F., Berglund, J. S., Tan, Y., Schneider, K. T., Cutler, M. W., Kalina, W. V., Simon, R., Hovius, J., van Dam, A. P., Gessner, B. D., Angulo, F. J., Edwards, J., Pilz, A., Begier, E., Stark, J. H., Dybova, L., … Bielska, D. (2027). Incidence of medically attended Lyme borreliosis across endemic regions of six European countries (BOLD): a prospective multicentre active surveillance study. The Lancet Regional Health – Europe, 72, Article 101907. https://doi.org/10.1016/j.lanepe.2026.101907

Image Credits: AI Generated

DOI: 10.1016/j.lanepe.2026.101907

Keywords: Lyme borreliosis, BOLD study, incidence, Europe, active surveillance, tick-borne disease, Borrelia afzelii, erythema migrans, Slovakia, serology, public health, vaccine development