A live vaccine against tick-born encephalitis: integrated studies. III. Response of man to a single dose of the E5 "14" clone (Langat virus).

The clinical and immunological events in volunteers following administration of 5 and 6 dex newborn mouse intracerebral LD5O (NmicLD5O) of the E5 "14" virus clone, segregated from the Langat TP21 E5 strain (tick-borne encephalitis-TE-complex) and propagated in 7 days old SPF chick embryos, are reported. The experimental vaccine, containing the cloned virus, carrying a set of genetic markers of an ic(+) sc s(+/-) t e u (s) N character, caused in volunteers no clinically recognizable effects, but elicited an immune response in them, as determined in PS-cell culture neutralization tests with various TE (western and eastern subtype) strains. In all vaccines, given the virus intramuscularly, seroconversions from negativity to positivity were observed, with antibody titres ranging from 4-128. The parenteral virus administration was superior to the oral route.