Using Sanofi Pasteur's data from the phase 3 clinical trials of Dengvaxia, which involved more than 35,000 children in 10 dengue-endemic countries, they showed that, during year 3 of the trial, among children ages 5 years or younger, 54.2% of whom were seronegative on enrollment, the dengue hospitalization rate was significantly higher among vaccinees than controls. The rate was 0.99% for the former (20 of 2,029) and 0.2% for the latter (2 of 1,005), a relative risk of 4.95, which translates into a fivefold risk of hospitalization.