Dengue is a common disease throughout Asia and the Pacific. Following the third dosage of a recombinant, live, attenuated, tetravalent dengue vaccine, participants in two phase II studies performed in Singapore and Vietnam were followed for up to four years (CYD-TDV). Three doses of CYD-TDV or a placebo were administered to participants at 0, 6, and 12 months. In both trials, dengue plaque reduction test antibody titers were assessed. The number of antigen-specific CD4+ and CD8+ T-cells that produce cytokines was measured in Singapore in order to determine cell-mediated immunity (CMI). In addition, post-hoc analyses were performed on individuals aged > 9. During long-term follow-up, related and fatal severe adverse events (SAEs) were recorded. Participants in Singapore and Vietnam who got 1 CYD-TDV injection completed long-term follow-up in 87 percent and 92 percent of cases, respectively. Geographic mean titers at four years in Vietnam and Singapore were between 30.2 and 73.7 dil/dil, respectively. There were still detectable amounts of Interferon and Interleukin-13 four years after immunization. The levels of tumor necrosis factor- at four years were similar to those at three years.

Participants who were seropositive at baseline had greater seropositivity rates at year four than those who were seronegative. There were no safety issues found throughout the investigation. It was immunogenic for four years following the third dosage of CYD-TDV.