WATCH: De Blasio says vaccine passports are 'the shape of things to come'

New York Mayor Bill de Blasio doubled down on his plans for a "Key to NYC" vaccine passport, which will require New Yorkers to show proof of vaccination in order to dine out, go to the gym, or attend cultural events.

That policy was criticized by Boston's mayor, who said it was reminiscent of slavery.

De Blasio was asked by a reporter about his assumption that other localities in the US want to "emulate Key to NYC pass. Can you give us more details?" The reporter asked if any leaders or officials had reached out to him from other places.

De Blasio declined to give details, out of "respect," he said, for how those placed have to govern and how they go about doing things. "But there's definitely been a lot of interest from public and private sector," he said.

De Blasio added that people are "unquestionably going to see a lot more" of vaccine passports in the near future.

"I announced the key to NYC pass and about five hours later, the President of United States endorsed it," said de Blasio. He thanked the President, saying "that was very important and very helpful."

De Blasio said the restaurant industry is happy about the vaccine passports, too. "We need government to tell us this is the right thing to do. So we have a clear standard we can point to, and I think you're going to see a lot of people embrace it," added de Blasio.

He noted that there would be some opposition, but "overwhelmingly, this is the shape of things to come, and more and more people in the public and private sector are going to use this approach to make sure folks are vaccinated.

The NYC mayor also addressed criticism from Boston's Democrat mayor Kim Janey, who said the vaccine passport is a "reminiscent of slavery".

"I am hoping and praying she hasn't heard the detail and has been improperly briefed because those statements are absolutely inappropriate," said de Blasio.

"This is an idea of how to save lives," he added. "I'm assuming the interim mayor has not heard the whole story because I can't believe she would say it's okay to leave so many people unvaccinated."

New York City's vaccine passport will be implemented in two phases. The first phase will include the rollout and will begin on August 16, with the second phase being enforcement will start on September 13.