In a number of dengue-endemic countries in Asia and Latin America, the tetravalent dengue vaccine (CYD-TDV; Dengvaxia®) is given in three doses, six months apart, to children aged nine. Participants in the CYD63 (NCT02824198) trial were randomized 3:1 to receive a booster CYD-TDV dosage (Group 1) or placebo. They had previously received three doses of CYD-TDV in the CYD28 study more than 5 years prior (Group 2). The geometric mean titres of dengue neutralizing antibodies (PRNT50 GMTs) for each of the four dengue serotypes were determined in sera taken before and 28 days after booster injections. For each serotype, non-inferiority of the booster immune response to that elicited after the third dosage was proven if the lower limit of the two-sided 95 percent confidence interval (CI) for GMT ratios (GMTRs) between post-booster CYD-TDV dose and post-dose 3 in Group 1 was >0.5. Overall, 118 participants received either a CYD-TDV booster or a placebo, and 116 finished the trial; two participants were removed due to noncompliance. GMTs in the booster CYD-TDV group increased by 1.74- (serotype 1) to 3.58-fold post-booster injection across all serotypes (serotype 4).

There were no obvious gains in the placebo group. Serotypes 1, 3, and 4 were shown to be non-inferior, while serotype 2 was not. There were no safety concerns discovered. These findings indicate that a CYD-TDV booster administered 5 or more years later tended to restore GMTs to pre-dose 3 levels.