New York City Mayor Bill de Blasio (D) on Monday announced a new COVID vaccine mandate for all private companies.

Why it matters: De Blasio, speaking on MSNBC's "Morning Joe," said the "first-in-the-nation measure" will go into effect starting Dec. 27. It will apply to approximately 184,000 businesses.

• There is no weekly testing option.
• The city had already imposed a vaccine mandate for city workers.

Details: De Blasio also said children between the ages of 5 and 11 will be required to show proof of vaccination to access indoor dining, as well as fitness and entertainment venues starting Dec. 14. Proof of at least one vaccine dose will be enough to allow kids to participate in those activities.

• Kids in this age group will also need to get vaccinated to participate in extracurricular activities, such as sports, band, orchestra and dance.
• New Yorkers aged 12 and older must now show they have had two doses of the vaccine, except for those who have received the Johnson & Johnson vaccine.

What he's saying: "We in New York City have decided to use a preemptive strike, to really do something bold to stop the further growth of COVID and the dangers it’s causing to all of us," de Blasio said.

• "So as of today, we’re going to announce a first-in-the-nation measure. Our health commissioner will announce a vaccine mandate for private sector employers across the board."
• The mayor said his office will work "with the business community" to enforce the mandate.

Between the lines: The measure is set to take effect just days before the end of de Blasio's term.

• A spokesperson for Eric Adams, who takes office on Jan. 1, told the New York Times that the "mayor-elect will evaluate this mandate and other Covid strategies when he is in office and make determinations based on science, efficacy and the advice of health professionals.”

The big picture: In September, President Biden announced mandates for employers that required over 80 million Americans working in the private sector to receive a COVID-19 vaccine or get tested weekly.

• The federal mandate has been stalled as legal challenges make their way through the courts.

What to watch: The city will issue "additional enforcement and reasonable accommodation guidance" on Dec. 15, according to a press release.

Editor's note: This story has been updated with additional details throughout.