Abstract. rDEN4D30-4995 is a live attenuated dengue virus type 4 (DENV4) vaccine candidate specifically designed as a further attenuated derivative of the rDEN4D30 parent virus. In a previous study, 5 of 20 vaccinees who received 10 5 plaque-forming units (PFU) of rDEN4D30 developed a transient elevation of the serum alanine aminotransferase (ALT) level and an asymptomatic maculopapular rash developed in 10 of 20. In the current study, 28 healthy adult volunteers were randomized to receive 10 5 PFU of rDEN4D30-4995 (20) or placebo (8) as a single subcutaneous injection. The vaccine was safe, well-tolerated, and immunogenic. An asymptomatic generalized maculopapular rash and elevations in ALT levels were observed in 10% of the rDEN4D30-4995 vaccinees. None of the rDEN4D30-4995 vaccinees became viremic, yet 95% developed a four-fold or greater increase in neutralizing antibody titers. Thus, rDEN4D30-4995 was demonstrated to be safe, highly attenuated, and immunogenic. However, an asymptomatic localized erythematous rash at the injection site was seen in 17/20 rDEN4D30-4995 vaccinees. Therefore, alternative DENV4 vaccine strains were selected for further clinical development.

With the steady rise in tick-borne encephalitis virus (TBEV) infections in Europe, development of a live attenuated vaccine that will generate long-lasting immunity would be of considerable benefit. A chimeric flavivirus, designated LGT/DEN4, was previously constructed to have a genome containing the prM and E protein genes of Langat virus (LGT), a naturally attenuated member of the TBEV complex, and the remaining genetic sequences derived from dengue 4 virus (DEN4).LGT/ DEN4 was highly attenuated in rodents and non-human primates, and clinical trials in humans were initiated. Twenty-eight healthy seronegative adult volunteers were randomly assigned in a 4:1 ratio to receive 10 3 PFU of LGT/DEN4 or placebo. Volunteers were closely monitored for clinical responses and for blood chemistry and hematological changes, and the level of viremia and the magnitude and duration of the neutralizing antibody response were determined. The LGT/DEN4 vaccine was safe and viremia was seen in only one vaccinee. Infection induced a neutralizing antibody response to wild-type LGT in 80% of volunteers with a geometric mean titer (GMT) of 1:63 present on day 42 post-immunization; however the antibody response against TBEV was both much less frequent (35%) and lower in magnitude (GMT = 1:9). To assess the response to a booster dose, 21 of the original 28 volunteers were re-randomized to receive a second dose of either 10 3 PFU of vaccine or placebo given 6 to 18 months after the first dose. The immunogenicity against either LGT or TBEV was not significantly enhanced after the second dose of vaccine. Thus, chimerization of LGT with DEN4 yielded a vaccine virus that was highly attenuated yet infectious in humans. The level of replication was sufficiently restricted to induce only a weak cross-reactive antibody response to TBEV. To provide a sufficient level of immunity to widely prevalent, highly neurovirulent strains of TBEV in humans, vaccine candidates will likely need to be based on the TBEV structural protein genes.