PHILADELPHIA — More than eight months ago, large studies found that both the Pfizer-BioNTech and Moderna COVID-19 vaccines reduced the risk of illness by more than 90%. Yet the U.S. Food and Drug Administration has not yet granted them full approval, to the dismay of public-health officials eager to boost vaccination rates as the delta variant sends infections skyward. So what’s the holdup? Part of the reason, according to experts in drug regulation, may be simply that the agency can afford to take the time. In December, the FDA authorized the first two vaccines for emergency use, and did so f...