Tick-borne encephalitis vaccine failures – review of the literature and 12 years population based data

The aim of the study was to access clinical features and laboratory characteristics of Tick-borne encephalitis (TBE) vaccine failures. This study was a retrospective, intensive search for TBE cases from all hospitals in Latvia. TBE cases of any age were identified as reported to the Centre of Disease Prevention and Control of Latvia from 2007 through to 2018 and combined with additional data derived from patient’s case histories/medical records in Latvian hospitals. These cases were categorized by TBE vaccination history (i.e. vaccinated vs non-vaccinated). Additional PubMed search was conducted to identify published cases of TBE vaccine failures through to December 2019.  A total of 3,106 TBEV-infections were identified in Latvia during the 12 study years. A total of 58 cases (1.8%) had received at least one prior TBE vaccine dose. The mean age of vaccinated TBE cases (34 years) was significantly lower than in non-vaccinated cases (50 years). When comparing TBE disease severity in vaccinated and non-vaccinated groups: mild cases (non-CNS TBE) were relatively more frequent among TBE vaccinated cases (23/58; 39%) than non-TBE-vaccinated cases (782/3027; 25%).Additional  literature review identified 534 TBE cases with prior vaccination history. Mostly, TBE cases with central nervous system disease were reported. Results of TBEV specific IgM and/or IgG antibodies of initial serum were reported in 93/534 patients (17.4%), of them 7.5% were detected as vaccine non-responders and 62.4% with absent/low initial specific TBEV IgM antibody response. The population-based analysis and data presented here indicate that TBE vaccination is highly effective. Our findings suggest that nevertheless vaccine failure, TBE vaccination has the potential to reduce overall disease severity after infection. Possible immunological and clinical factors in vaccinated TBE patients that may negatively influence vaccine effectiveness and lead to vaccine failure are not well investigated, but extremely needed.