Routine childhood vaccinations dropped dramatically during the early months of the coronavirus pandemic, and although they began rebounding last summer as families rescheduled doctors’ visits, many children and adolescents are behind on their shots, according to a federal health report released Thursday. The lag might pose “a serious public health threat” of outbreaks of vaccine-preventable illnesses, such as measles and whooping cough, that have the potential to derail school-reopenings, according to the CDC.