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New York City Mayor Bill de Blasio said Monday that he is requiring all New York City employees to be vaccinated – or tested once per week for COVID-19 – by Sept. 13, and said the city will step up its enforcement of its mask mandate for unvaccinated workers, threatening "very tough" consequences. 

De Blasio also announced that New York City is pushing Facebook, Twitter and Instagram to deplatform the 12 individuals who one study alleged are the most prolific spreaders of vaccine disinformation. He said that in a letter to Mark Zuckerberg and Jack Dorsey, he will convey that "if you don't stop the lies then you are complicit in the spread of COVID."

The comments come as de Blasio has avoided reinstating a universal mask mandate in his city as some other liberal enclaves – like Los Angeles and Provincetown, Massachusetts – have done in light of the delta variant. The mayor instead has pushed vaccination as the best way to squash the pandemic.

"On Sept. 13 the entire city workforce will be mandated under the COVID safety mandate to either get vaccinated, which is far preferable, or get tested once a week," de Blasio said. "This is about our recovery… this is about keeping people safe."

MASKS FOR VACCINATED GAINING MOMENTUM AS MORE CITIES, STATES REQUIRE OR RECOMMEND FACE COVERINGS

The mayor cited school returning in September, major companies bringing back their employees to New York City offices and the return of many residents who leave town for the summer as reasons for the timing of the new rule. 

"On Sept. 13 which is the first full day of school, every single city employee will be expected to be either vaccinated or be tested weekly," de Blasio added. 

De Blasio also announced that New York will step up its enforcement of its mask mandate for unvaccinated city employees, beginning Monday. He said that will include "very tough… consequences" for unvaccinated people who he called a "threat" to transmit the virus. 

"Let's be blunt. If you are a city employee and you're unvaccinated you must wear a mask indoors at work. We will not tolerate any decision to do otherwise because this is about protecting peoples' health and well being," de Blasio said. 

Commissioner of Labor Relations Renee Campion said mask-wearing for unvaccinated people will be "a job requirement."

"We will expect employees to comply," Campion added. "If employees refuse to comply, they just can't be at work. And in fact they will not be paid."

Mayor of New York City Bill de Blasio speaks during the opening of a vaccination center for Broadway workers in Times Square on April 12, 2021 in New York City. De Blasio announced that NYC workers will be required to be vaccinated or tested weekly for COVID-19, and foreshadowed potentially stricter requirements in the future.  (Photo by Noam Galai/Getty Images)

AFSCME AFL-CIO District Council 37, which is the largest public employee union in New York City, pushed back against the mayor's vaccine or weekly testing requirement. 

"If City Hall intends to test our members weekly, they must first meet us at the table to bargain," Executive Director Henry Garrido said in a statement. "While we encourage everyone to get vaccinated and support measures to ensure our members’ health and wellbeing, weekly testing is clearly subject to mandatory bargaining. New York City is a union town and that cannot be ignored."

If people would like to remain unmasked at work, de Blasio said, they need to "immediately go get vaccinated."

Outside of the city's purview, meanwhile, de Blasio encouraged private employers to go at least as far as the city is in mandating vaccines or weekly testing. But he said it would be even better if employers who "have a lot of freedom" to "set their own rules" mandate vaccination without exception.

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The mayor foreshadowed that the city's vaccine and virus-related measures could get tougher. 

"We're going to keep climbing this ladder and adding additional measures as needed," he said. "Mandates and strong measures whenever needed to fight the delta variant."

De Blasio also announced that New York City is rolling out a vaccine passport app, called the "NYC COVID Safe app." He compared it to New York state's Excelsior app.