Starting August 2, all NYC Health + Hospitals staff and health department clinical workers must either show one time proof of vaccination or a weekly negative COVID test.
De Blasio said they are "trying to strike a balance" by offering employees a choice.
"Up to now, things have been entirely voluntary," he said. "They are no longer voluntary. You have two choices."
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The mayor was asked several times whether other unvaccinated city workers will soon also have to option to make the same choice, and he said it is under consideration.
"Reaching other municipal workers, we are going to be looking at that," he said. "We are definitely looking at other possibilities, but we are not there yet. This is the piece we are doing right away."
Only 60% of New York City health employees are already vaccinated, which is nonetheless higher than the national average for health care workers of about 50%. Still, the mayor said he expects pushback.
"I'm sure (unions) will have their concerns, but I think there is cognizance we are in an urgent situation, and we are trying to strike a balance."
George Gresham, president of 1199SEIU United Healthcare Workers East, which represents registered nurses, said he was in favor of the proposal.
"We commend Mayor de Blasio's position of not making vaccination a condition of employment," he said. "The health and safety of our members and their patients remains our top priority, which is why we encourage our members to get vaccinated. However, there are other cooperative avenues that can be taken to address the concerns of both labor and management. Threatening our members' freedom of choice and livelihood is not one of them."
City Health Commissioner Dr. David Chokshi said he expected the plan to be expanded beyond the clinics in the weeks to come.
"The simple fact is that if you are vaccinated, every activity is safer," he said. "Vaccination has been and continues to be the single most important precaution we can take when dealing with the public and our colleagues."
Employment attorney and small business expert Jon Bell, founder of Bell Law Group, said that vaccine mandates for businesses are legal.
"Both private sector and public sector can require testing for unvaccinated people," he said. "Unvaccinated people are not a protected class. So in other words, you can't make different rules and laws for people based on their race, age, religion, gender, disability, protected classes, things of that nature. Unvaccinated is not. It's not protected...They could even go a step further and state if you're not vaccinated, you can't come to work at all. So even allowing them to come to work unvaccinated and require that they undergo...testing is not only legal, it seems to be the right thing to do."
He also said termination of employment for failure to comply is legal as well.
"If an employee refuses the requirement of the employer that's legal, like it is, in the sense, the employer can turn around and terminate the employee," he said. "And the employee really will be left without recourse."
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More than 1,000 people a day are now testing positive in New York, based on the latest seven-day average, according to AP's analysis of state data. That's up from a pandemic low of 306 per day on June 25.
About 56% of New York's 20 million residents are fully vaccinated, a small increase from 54.5% as of July 4, despite efforts by Gov. Andrew Cuomo's administrations to increase rates in the least vaccinated parts of the state.
Vaccination rates are lowest in rural counties in western and central New York, as well as parts of New York City including the Bronx, Brooklyn and Staten Island.
(The Associated Press contributed to this report.)
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