Santonja et al.
Tick-borne encephalitis in vaccinated patients: a retrospective case-control study and analysis of vaccination field effectiveness in Austria from 2000 to 2018
J Infect Dis. 2022;jiac075. doi: 10.1093/infdis/jiac075. Online ahead of print.

Despite high field effectiveness (FE) of TBE vaccines licensed in Europe (FSME Immun and Encepur), TBE cases are rarely reported in vaccinated individuals, and the rate of vaccination failures is more likely in populations with a high vaccination coverage like Austria. Some studies have indicated that TBE in vaccinated persons could have a more severe disease course.

Therefore, a study has been carried out to determine the proportion of mild and severe forms of TBE in vaccinated and matched non-vaccinated persons. A retrospective longitudinal study of all hospitalized patients with TBE in Austria between 2000 and 2018 was performed. The study included 1,545 hospitalized patients of whom 206 had been vaccinated before, while 1,245 patients never had received a TBE vaccination (for 94 patients, the vaccination status was not available).

In both groups, the male-female ratio was similar, the majority of patients were older than 50 years of age, but the vaccinated patients were significantly younger, and the proportion of patients <17 years old was higher among vaccinated (19.9%) than among non-vaccinated patients (10.0%).

Most of the vaccinated patients were incompletely vaccinated (N = 142, 68.9%) and only 64 patients (39.8%) were regularly vaccinated.

The proportion of severe relative to mild disease was higher in vaccinated patients (57.8% vs. 39.2% in non-vaccinated patients). This difference was observed in all age groups but reached statistical significance only in the youngest age group (1–16 years) and in those ≥60 years.

The incidence of severe and mild disease forms in the vaccinated and non-vaccinated populations in all three age groups has been analyzed. The incidence of TBE in the Austrian population was much lower among those vaccinated (0.16–0.23 TBE cases per 100,000 vaccinated population) than among non-vaccinated population (5.08–5.47 cases/100,000. This marked difference was observed for both mild and severe disease in all age groups. The overall FE for all clinical cases for persons ≥1 year old was >95%. It was >94% against mild disease in all age groups and severe disease in those ≥17 years.

Nevertheless, the calculated FE against severe disease among those 1–16 years of age was lower (82.7%) than for adults, indicating that the current pediatric vaccine dose might be insufficient to prevent disease in some cases. The FE against all clinical courses in the younger group was 91.2%–92.7%.

In summary, this study shows that TBE vaccination is highly effective and TBE vaccinees of all age groups have a markedly reduced risk to acquire TBE compared to non-vaccinated individuals.

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