Tick-borne Encephalitis Vaccine Failures: A 10-year Retrospective Study Supporting the Rationale for Adding an Extra Priming Dose in Individuals Starting at Age 50 Years.

BACKGROUND:Southern Sweden is endemic for tick-borne encephalitis (TBE), with Stockholm County as one of the high-risk areas. Our aim in this study was to describe cases of vaccine failures and to optimize future vaccination recommendations. METHODS:Patients with TBE were identified in the notification database at the Department of Communicable Disease Control and Prevention in Stockholm County during 2006-2015. Vaccine failure was defined as TBE despite adherence to the recommended vaccination schedule with at least 2 doses. Clinical data were extracted from medical records. RESULTS:A total of 1004 TBE cases were identified, 53 (5%) were defined as vaccine failures. In this latter group, the median age was 62 years (6-83). Forty-three (81%) patients were aged >50 years and 2 were children. Approximately half of the patients had comorbidities, with diseases affecting the immune system accounting for 26% of all cases. Vaccine failures following the third or fourth vaccine dose accounted for 36 (68%) of the patients. Severe and moderate TBE disease affected 81% of the cases. CONCLUSIONS:To our knowledge, this is the largest documented cohort of TBE vaccine failures. Vaccine failure after 5 TBE vaccine doses is rare. Our data provide rationale for adding an extra priming dose to those aged ≥50 years.