A live vaccine against tick-borne encephalitis: integrated studies. I. Basic properties and behaviour of the E5 "14" Clone (Langat virus).

The course of plaque segregation from the Langat E5 strain of the E5 "14" clone is described. The virus, displaying an ic+ sc s plus or minus t e u s character, reproduced at 35 and 37, but not at 39 degree C. In subcutaneously (sc) inoculated monkeys, viraemia lasted for 2--4 days without detectable central nervous system (CNS) involvement. In sc inoculated 8--10 g mice, the infection was manifested only by a transient trace viraemia, but caused a marked resistance against challenge with virulent tick-borne encephalitis (TE) viruses, protection indices being 5.7--6.7. Pathogenetic investigations in challenged, live virus-immunized mice showed no signs of a marked productive infection except of a booster effect.