New York City Mayor Bill de Blasio said a third Pfizer booster shot is available to eligible New Yorkers, "literally right now."

"As of now, as of this exact moment, New Yorkers in a number of categories are eligible for the 3rd booster shot, Pfizer only, for the COVID vaccine," the mayor told radio station WNYC Friday.

Eligible New Yorkers include anyone who got their second shot six months ago and are 65 or older; in a long-term care facility or nursing home; are between 18 and 64 years old with an underlying medical condition; or are between 18 and 64 years old and a front-line or health care worker doing direct work with the public, the mayor said.

On Thursday, the Centers for Disease Control and Prevention  an independent advisory panel's recommendation for seniors and other medically vulnerable Americans to get a booster shot of Pfizer's COVID-19 vaccine six months after their second dose.

"Literally now you can go online, vax4nyc, either make an appointment right now for the coming days or you can get a list of all the city-run sites and you can walk in today if you are in those categories," de Blasio said.

-ABC News' Aaron Katersky

The U.S. Centers for Disease Control and Prevention has endorsed an independent advisory panel's recommendation for seniors and other medically vulnerable Americans to get a booster shot of Pfizer's COVID-19 vaccine, six months after their second dose.

Dr. Rochelle Walensky, director of the CDC, also partially overruled her agency's advisory panel in a notable departure by adding a recommendation for a third dose for people who are considered high risk due to where they work, such as nurses and teachers -- a group which the panel rejected in its recommendation. Some panelists said that without further data, they weren't comfortable with automatically including younger people because of their jobs.

In a statement announcing her decision late Thursday, Walensky pointed to the benefit versus risk analysis she had weighed, and data rapidly evolving.

"In a pandemic, even with uncertainty, we must take actions that we anticipate will do the greatest good," Walensky said. "While today’s action was an initial step related to booster shots, it will not distract from our most important focus of primary vaccination in the United States and around the world."

With Walensky's final sign-off, booster shots will now quickly become available for millions more Americans at pharmacies, doctors' offices and other sites that offer the Pfizer vaccine as soon as Friday.

-ABC News' Sasha Pezenik

The nation’s largest union of registered nurses pushed back against the U.S. Centers for Disease Control and Prevention advisory panel's vote on COVID-19 booster shots, calling not including front-line workers like nurses in its recommendations "unconscionable."

National Nurses United is urging CDC Director Rochelle Walensky to bypass what the advisory panel, ACIP, recommended and add nurses and other health care workers to the list of eligible booster recipients.  

"Nurses and other health care workers were among the first to be vaccinated because of their high risk of exposure to the virus," Deborah Burger, the union's president, said in a statement. "Why leave them out of booster shots?"

“It is unconscionable that ACIP would not vote to keep us safer from death, severe Covid, and long Covid,” Burger continued. “We must do everything possible to ensure that the health of our nurses and other health care workers will not be put even more at risk."

ACIP voted Thursday to recommend a third Pfizer dose for people aged 65 and older, as well as those as young as 18 if they have an underlying medical condition.

In its authorization Wednesday, the Food and Drug Administration did agree to make the shots available to front-line workers. But ACIP said there was not yet enough data to support providing booster shots automatically to young people because of their jobs.

-ABC News' Sasha Pezenik

An independent advisory panel for the U.S. Centers for Disease Control and Prevention unanimously voted on Thursday to recommend a booster shot of Pfizer's COVID-19 vaccine for seniors and other medically vulnerable Americans, six months after their second dose.

The panel voted to allow a third dose for people aged 65 and older, along with long-term care facility residents and people as young as 18, if they have an underlying medical condition. People younger than 49, however, should only get that booster shot if the benefits outweigh the risks, the panel said -- a personal consideration to discuss with their doctor.

The panel voted "no" for a booster for those in an occupational or institutional setting where "the burden of COVID-19 infection and risk of transmission are high," such as nurses and teachers. Some panelists said that without further data, they weren't comfortable with automatically including younger people because of their jobs.

-ABC News' Sasha Pezenik, Anne Flaherty, Sony Salzman and Arielle Mitropoulos

The CDC’s independent advisory panel is set to vote around 3 p.m. ET on which Americans are eligible now for a Pfizer booster.

After the vote, CDC director Rochelle Walensky is expected to weigh in with her official endorsement. The CDC is not bound by the panel's recommendations but usually follows it. State officials may also implement their own criteria.

he FDA granted authorization Wednesday to the following groups: Anyone 65 or older as well as people as young as 18 if they have a medical condition that puts them at risk of severe COVID-19 or if they work a frontline job that makes it more likely that they would get infected. After authorization Wednesday night, the FDA's acting commissioner Dr. Janet Woodcock said some of the groups that could be classified as front-line workers are health care employees, teachers and grocery store staffers, as well as people in prisons and homeless shelters.

-ABC News' Sasha Pezenik