In November 2017, Sanofi acknowledged that some participants were put at risk of severe dengue if they had no prior exposure to the infection; subsequently the Philippine government suspended the mass immunization program with the backing of the WHO which began a review of the safety data.Phase III trials in Latin America and Asia involved over 31,000 children between the ages of two and 14 years. In the first reports from the trials, vaccine  efficacy was 56.5% in the Asian study  and 64.7% in the Latin American study in patients who received at least one injection of the vaccine. Efficacy varied by serotype. In both trials vaccine reduced by about 80% the number of severe dengue cases. An analysis of both the Latin American and Asian studies at the 3rd year of follow-up showed that the efficacy of the vaccine was 65.6% in preventing hospitalization in children older than nine years of age, but considerably greater (81.9%) for children who were seropositive (indicating previous dengue infection) at baseline. The vaccination series consists of three injections at 0, 6 and 12 months. The vaccine was approved in Mexico, Philippines, and Brazil in December 2015, and in El Salvador, Costa Rica, Paraguay, Guatemala, Peru, Indonesia, Thailand and Singapore in 2016. Tradenamed Dengvaxia, it is approved for use for those aged nine and older and can prevent all four serotypes.
This led to a controversy in the Philippines where more than 733,000 children and more than 50,000 adult volunteers were vaccinated regardless of serostatus.The World Health Organization (WHO) recommends that countries should consider vaccination with the dengue vaccine CYD-TDV only if the risk of severe dengue in seronegative individuals can be minimized either through pre-vaccination screening or recent documentation of high seroprevalence rates in the area (at least 80% by age nine years).The WHO updated its recommendations regarding the use of Dengvaxia in 2018, based on long-term safety data stratified by serostatus on 29 November 2017. Seronegative vaccine recipients have an excess risk of severe dengue compared to unvaccinated seronegative individuals. For every 13 hospitalizations prevented in seropositive vaccinees, there would be 1 excess hospitalization in seronegative vaccinees per 1,000 vaccinees. WHO recommends serological testing for past dengue infection In 2017, the manufacturer recommended that the vaccine only be used in people who have previously had a dengue infection as otherwise there was evidence it may worsen subsequent infections. The initial protocol did not require baseline blood samples prior to vaccination in order to establish an understanding of increased risk of severe dengue in participants who had not been previously exposed.