Dengue 2 vaccine: dose response in volunteers in relation to yellow fever immune status.

A live dengue 2 vaccine was tested in 38 volunteers in an evaluation of the safety, infectivity, and immunogenicity of doses of 10(1.8)-10(5.5) plaque-forming units. Twenty yellow fever-immune and 18 yellow fever-nonimmune individuals received 0.5 ml of vaccine sc. Immunization was dose related in yellow fever-immune volunteers, with a 50% immunizing dose of 10(3.3) plaque-forming units. In the group not immune to yellow fever, some but not all recipients of each vaccine dilution were immunized, and no 50% immunizing dose could be estimated. Volunteers immune to yellow fever developed adequate titers of neutralizing antibody to dengue 2 virus and maintained them for at least three years; those not immune to yellow fever developed lower antibody titers that disappeared within six months in half of the cases. More than 40 isolates of dengue 2 virus from 12 volunteers retained the in vitro growth characteristics of the vaccine virus; this result affirmed the genetic stability of the virus. Common clinical signs in immunized individuals were leukopenia (55%), macular rash (15%), and fever (10%).