Further virological and clinical investigations on the attenuated E5"14" virus from the tick-borne excephalitis complex.

A total of 22 persons, given a single intramuscular dose of 3.1 times 10(6) newborn mouse ic LD50/ml of the E5"14" virus (stored for 26 months at 4 degrees C in lyophilized state), were subjected to rigorous clinical, clinical-laboratory and virological investigations. Clinical observations and laboratory tests (blood and cerebrospinal fluid cytology and biochemistry) revealed no ill-effects or deviations attributable to the immunization procedure. After administration of the immunizing dose, no viraemia was detected and specific seroconversions from negativity to positivity were found after 9 weeks in 73-82% of the subjects, using different tick-borne encephalitis (western subtype) viruses in the virus neutralization experiments. The results obtained seem to give increased confidence in the further use of this attenuated virus clone with strictly defined characteristics.