New York City will become the first city in the United States to require proof of vaccination for a variety of indoor activities for both employees and customers, Mayor Bill de Blasio announced on Tuesday, marking the city's latest effort to combat its coronavirus outbreak.

The new mandate, called the Key to NYC Pass, will go into effect Aug. 16, with the city to begin inspecting businesses for compliance in mid-September. The order is designed to increase immunization rates throughout the city in order to curb the spread of the highly contagious Delta variant.

"The key to New York City--when you hear those words, I want you to imagine the notion that because someone's vaccinated, they can do all the amazing things that are available in this city. This is a miraculous place literally full of wonders," de Blasio said when announcing the Key to NYC Pass. "If you're vaccinated, all that's going to open up to you. But if you're unvaccinated, unfortunately you will not be able to participate in many things."

The new initiative will require vaccination for workers and customers in indoor dining, gyms, and entertainment facilities to present proof of at least one dose of either the Pfizer (PFE  )-BioNTech (BNTX  ), Moderna (MRNA  ) or Johnson & Johnson (JNJ  ) vaccine for entry. Employees and patrons can show proof of vaccination with either their physical card or New York State's Excelsior app.

The Key to NYC Pass initiative comes soon after de Blasio declined to reintroduce an indoor mask mandate in the city, with the mayor instead encouraging vaccination and incentivizing vaccines with a $100 cash offer. The city hopes that limiting indoor activities to only the vaccinated will help increase the city's vaccinated population and reduce tourism from unvaccinated visitors that pose a risk of spreading the Delta variant.

The city's vaccine requirement shows a new strategy in New York's ongoing effort to mitigate infections in the city that was once the epicenter of the pandemic about a year and a half ago. Roughly two-thirds of adults in NYC are fully vaccinated, according to city data, but there are still neighborhoods without the city with lower rates.

"Not everyone is going to agree with this, I understand that," de Blasio said. "But, for so many people, this is going to be the lifesaving act--that we're putting a mandate in place, it's going to guarantee a much higher level of vaccination in this city, and that is the key to protecting people and the key to our recovery."

"That is why it's the Key to NYC," de Blasio added. "The Key to NYC pass opens a lot of doors and we need it."

The final list of the types of businesses including in the new mandate has yet to be finalized.