BANGKOK—Dispatches from the frontlines of the war on dengue are growing more and more dispiriting. In September 2012, pharmaceuticals giant Sanofi Pasteur revealed that a vaccine against the centuries-old tropical malady had stumbled in clinical trials. Meanwhile, dengue's toll is heavier than thought. According to an April report based on modeling, the annual global incidence, close to 390 million, is about three times higher than the number of cases estimated by the World Health Organization (WHO) for 2009. At a meeting here earlier this week, scientists described the latest blow on the battlefield: a fifth dengue virus. The first new serotype discovered in a half century could complicate control efforts, especially the quest for a vaccine that protects against all types simultaneously.