“People crowded on the subway to get to the parade or on Metro-North for example, people going to a bar afterward,” Mr. de Blasio said. “That’s what we are assessing right now.”

Neil Cosgrove, an official with the Ancient Order of Hibernians, an Irish diaspora group, who was not involved in discussions over the parade, said that the organizers had been looking to the city for guidance.

“My understanding is there is a bit of a tango going on between the parade committee and the city,” said Mr. Cosgrove on Wednesday before the cancellation was announced. “I think the city doesn’t want to pull the trigger, they want the parade committee to pull the trigger.”

On Wednesday, Gov. Andrew Cuomo said he had spoken to Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, whose “strong recommendation” was to “reduce large gatherings” like the parade.

“Why would you risk bringing thousands of people together knowing this is a virus that is easily communicable?” Mr. Cuomo said at a news conference. “St. Patrick’s Day is one of the great convenings of a large number of people.”

“If you listen to the experts, they are saying you should not have a St. Patrick’s Day convening at this time, which I believe makes sense,” the governor said.

St. Patrick is the patron saint of both Ireland and the Roman Catholic Archdiocese of New York, the second-largest diocese in the United States, which has traditionally played a role in the parade. Each year, the parade is reviewed by Cardinal Timothy Dolan of New York from the steps of St. Patrick’s Cathedral as it marches past.