“Flu was coming down, and then you saw this new ominous spike. And it was Covid. And it was spreading widely in New York City before anyone knew it,” said Dr. Thomas R. Frieden, the former head of the Centers for Disease Control and Prevention and former commissioner of the city’s Health Department. “You have to move really fast. Hours and days. Not weeks. Once it gets a head of steam, there is no way to stop it.”

Dr. Frieden said that if the state and city had adopted widespread social-distancing measures a week or two earlier, including closing schools, stores and restaurants, then the estimated death toll from the outbreak might have been reduced by 50 to 80 percent…

“Everything was slow,” said Councilman Stephen T. Levin, a Brooklyn Democrat who had called for City Hall to take swifter action as the outbreak spread. “You have to adapt really quickly, and nothing we were doing was adapting quickly.”…

“Every action I took was criticized at the time as premature,” Mr. Cuomo said in an interview. “The facts have proven my decisions correct.”

Mr. de Blasio said in a statement, “We’re dealing with a virus that’s only months old and science that changes by the day,” adding that “hindsight is a luxury none of us have in the heat of battle.”