New York City Mayor Bill de Blasio announced Wednesday that childhood vaccination rates have plunged amid the coronavirus pandemic, a veritable nosedive that threatens to render school reopenings in the fall impossible. For the period of March 23 to May 9, he said, vaccinations fell 63 percent compared to the same period last year, and by 91 percent for kids above the age of 2. Many pediatricians’ offices have closed during the pandemic, and parents may be wary of taking their kids to health-care centers, which have been hit hard by the contagion. The New York City school system is the country’s largest, with 1,800 schools and more than 1.1 million students. Vaccinations for preventable illnesses like measles and chickenpox are required in order to attend, and the city has seen outbreaks of those diseases in communities that refuse routine immunizations in recent years.