Categorization systems for tick-borne encephalitis virus (TBEV) infection lack consistency in classifying disease severity. To evaluate the need for a standard, consensus-based categorisation system for TBEV infection across subtypes, we gathered an expert panel of clinicians and scientists with diverse expertise in TBEV infection. Consensus was sought using the Delphi technique, which consisted of 2 web-based survey questionnaires and a final, virtual, consensus-building exercise. Ten panellists representing 8 European countries participated in the Delphi exercise, with specialities in neurology, infectious disease, paediatrics, immunology, virology, and epidemiology. Panellists reached unanimous consensus on the need for a standardised, international categorisation system to capture both clinical presentation and severity of TBEV infection. Ideally, such a system should be feasible for use at bedside, be clear and easy to understand, and capture both the acute and follow-up phases of TBEV infection. Areas requiring further discussion were (1) the timepoints at which assessments should be made and (2) whether there should be a separate system for children. This Delphi panel study found that a critical gap persists in the absence of a feasible and practical classification system for TBEV infection. Specifically, the findings of our Delphi exercise highlight the need for the development of a user-friendly classification system that captures the acute and follow-up (i.e., outcome) phases of TBEV infection and optimally reflects both clinical presentation and severity. Development of a clinical categorisation system will enhance patient care and foster comparability among studies, thereby supporting treatment development, refining vaccine strategies, and fortifying public health surveillance.

Bundeswehr Institute of Microbiology Munich Germany; Department of Infectious Diseases and Tropical Medicine University Munich Germany

Department of Infectious Diseases and Neuroinfections Medical University of Bialystok Żurawia 14 Białystok 15 540 Poland

Department of Infectious Diseases Epidemiology National Institute of Public Health Srobarova 49 Prague Czech Republic; Department of Epidemiology and Biostatistics 3rd Faculty of Medicine Charles University Ruska 87 Prague Czech Republic

Department of Infectious Diseases Institute of Biomedicine Sahlgrenska Academy at Gothenburg University Guldhedsgatan 10 41346 Gothenburg Sweden

Department of Infectious Diseases University Medical Centre Ljubljana Japljeva 2 1525 Ljubljana Slovenia

Department of Neurology Landesklinikum Mistelbach Gänserndorf Mistelbach Affiliated with Karl Landsteiner University of Health Sciences Krems Austria

Department of Pediatric Infectious Diseases Faculty of Medicine Masaryk University and Faculty Hospital Brno Czech Republic

Encephalitis International North Yorkshire United Kingdom; Institute of Infection Veterinary and Ecological Science University of Liverpool NIHR HPRU for Emerging and Zoonotic Infection Liverpool United Kingdom

Krankenhaus Nordwest Frankfurt Germany

Riga Stradins University Department of Paediatrics Children Clinical University Hospital Riga Vienibas gatve 45 Riga LV1004 Latvia

Rīga Stradiņš University Dzirciema St 16 Rīga LV 1007 Latvia; Rīga East University Hospital Hipokrata St 2 Rīga LV 1079 Latvia

RTI Health Solutions 3040 East Cornwallis Road Research Triangle Park NC United States

Vaccines and Antivirals Medical Affairs Pfizer Biopharma Group 500 Arcola Rd Collegeville PA 19426 United States

Vaccines and Antivirals Medical Affairs Pfizer Corporation Austria Vienna Austria

Vaccines and Antivirals Medical Affairs Pfizer Ltd Surrey United Kingdom

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