Steffen et al.
Can the booster interval for the tick-borne encephalitis (TBE) vaccine “FSME-IMMUN” be prolonged? – A systematic review.
Ticks Tick Borne Dis. 2021, 12(5):101779. doi: 10.1016/j.ttbdis.2021.101779

When the TBE vaccine FSME-IMMUN (based off the European virus subtype Neudörfl) was first licensed in 1976, specific TBE antibodies were the only surrogate marker for protection, and even forty years after its licensure there is still controversy about the duration of protection and if persistence of circulating TBE antibodies is the one and only mechanism that mediates protection. According to the label of the vaccine, booster doses should be applied every 3 to 5 years depending on age of the vaccinee, although a couple of studies have shown that antibodies persist much longer, and protection also remains when the recommended booster interval is overrun. So far, only in Switzerland and in Finland has the booster interval been increased to 10 years after the third or fourth dose (first booster dose) respectively. A systematic literature review has been conducted to analyze if the booster dose may generally be prolonged.

It is widely accepted that the mechanism of protection induced by TBE vaccination is mediated by circulating antibodies measured by ELISA and/or neutralization assay. However, there is no uniform threshold which can be used as a surrogate marker. Seropersistence studies carried out in a variety of European countries show that antibodies persist much longer than 3 to 5 years in the blood of vaccinees and a booster injection results in nearly 100% positive subjects. A prolongation of the booster interval to 10 years in Switzerland did not result in an increase of vaccine breakthroughs. Vaccine effectiveness is not impaired, and even among individuals with a history of irregular vaccinations the average protection remains high. Also, older individuals respond to a booster vaccination, although with lower antibody titers.

Analyses of the dynamic of the immune response after booster vaccination show that an increase in TBE antibodies can already be measured on day 7. This rapid immune response is due to immune memory which has been elicited by vaccination, and with an incubation period of usually 7 to 14 days after exposure to TBE virus, most infected persons can produce enough protective antibodies before the virus crosses the blood barrier to cause CNS involvement. With the evidence summarized in this review the authors recommend an extension of the TBE booster interval to 10 years after primary vaccination.

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