World Health Organization (WHO) chief Tedros Adhanom Ghebreyesus has said that the top health organisation has no guarantee whether any single Covid-19 vaccine now in development will “We have no guarantee that any single vaccine now in development will work, he said. “The more candidates we test, the higher the chance we will have of a safe and efficacious vaccine,” he added. According to the WHO, almost 200 vaccines for Covid-19 are currently in clinical and pre-clinical testing. “The history of vaccine development tells us that some will fail, and some will succeed,” Ghebreyesus said.