A rapid fluorescent focus inhibition test for detection of neutralizing antibodies to tick-borne encephalitis virus.

Tick borne encephalitis (TBE), is endemic in several countries in central and northern Europe, in the Baltic states and in Russia. Vaccination has been shown to lower efficiently the number of cases of this potentially very serious disease. However, the possibility to assess the neutralizing antibody response after clinical disease or vaccination has been hampered by the lack of easy and specific tests. We developed a rapid fluorescent focus inhibition test (RFFIT) and compared it with a standard plaque reduction neutralization test (PRNT) and an hemagglutination inhibition test (HI) for antibody detection in late convalescent sera from 18 patients with a previous clinical and serological diagnosis of TBE. Neutralizing titres obtained with RFFIT were almost identical to those obtained with PRNT, and correlated also well with the HI results. The RFFIT for detection of neutralizing antibodies to TBE-virus has an advantage over the standard PRNT in its easy and rapid performance (results are obtained in 1 vs 7 days), and over the HI in its specificity, since cross-reactions with other flaviviruses are minimized.