By CBS3 Staff

PHILADELPHIA (CBS/AP) — The U.S. moved a step closer Wednesday to offering booster doses of Pfizer’s COVID-19 vaccine to senior citizens and others at high risk from the virus as the Food and Drug Administration signed off on the targeted use of the extra shots.

The FDA authorized booster doses for Americans who are 65 and older, younger adults with underlying health conditions and those in jobs that put them at high-risk for COVID-19. The ruling represents a drastically scaled back version of the Biden administration’s sweeping plan to give third doses to nearly all American adults to shore up their protection amid the spread of the highly contagious delta variant.

President Joe Biden wanted boosters to be available for everyone this week. That’s not happening.

But who will be able to get a third shot, and when? That’s still very much up in the air.

A CDC advisory committee started two days of meetings to decide the specifics on who’s a high risk. For example, should health care workers get booster shots, and when?

“We’re living in a world right now that I call corrected science. We learn, we apply it, then we learn again,” said Dr. Michael Osterholm, director of the Center for Infectious Disease Research and Policy.

Pfizer says people who received a booster six to eight months after their second dose of the vaccine tripled their antibodies, increasing protection against mild infection and the contagious delta variant.

Since current discussions focus only on Pfizer, health officials have to decide what to tell Americans who received Moderna and Johnson & Johnson vaccines.

Those two companies have submitted booster data, but it hasn’t been reviewed yet.

“It’s messy, it’s ugly, but that’s because there’s scientific uncertainty and when there’s scientific uncertainty, we want to hear from all sides,” former White House senior adviser for COVID response Andy Slavitt said.

At the White House, Biden kicked off a virtual global vaccination summit. He said the U.S. is doubling its purchase of Pfizer’s vaccine to more than one billion doses to share with the world.

The president is also pressing other wealthy countries to do more.

“This is an all-hands-on-deck crisis,” Biden said.

The administration has said it’s able to provide extra vaccines to poor nations while at the same time increasing supplies here to cover booster shots.

CBS3’s Stephanie Stahl contributed to this report.

(© Copyright 2021 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)