Unvaccinated people 11 times more likely to die in COVID-19 delta surge

Dr. Jaqueline Miller, who presented data to the FDA committee on Thursday on behalf of Moderna, said the half dose works to boost antibody protections back up to the initial level.

"We chose the 50 microgram dose for the booster because we believe we should vaccinate with the lowest amount of antigen needed to induce an immune response at least equal to that in study 301 [the initial clinical study of Moderna vaccine], which was linked to vaccine efficacy of 93%," Miller said.

Miller added that "reducing the booster dose to 50 micrograms will also increase the worldwide vaccine supply of mRNA."

Americans who had a normal immune response to the first two shots and are getting a booster will need a half dose. But immunocompromised Americans -- people receiving cancer treatment or organ transplant recipients, for example -- need a full dose for their third shot to try and initiate the immune response they didn't get from the first two.

Moderna officials said they're preparing a letter explaining this administration to health care workers.

According to the FDA's review of Moderna's data, there was no evidence of increased side effects from booster doses.

-ABC News' Cheyenne Haslett

The U.S. is making progress in the fight against COVID-19 with daily cases and hospitalizations on the decline, but the nation must to do more to vaccinate Americans, President Joe Biden said Thursday.

The Labor Department will soon issue a vaccine mandate for workplaces with 100 or more employees, Biden said.

Biden called on more businesses to "step up" and more parents to get their children vaccinated when eligible.

"We can't let up now," he said.

The FDA's independent committee is meeting on Thursday and will vote in hours on whether to greenlight the Moderna booster for: people 65 and older; people 18 and older who are at high risk of severe illness because of an underlying health condition; and people 18 and older whose job may put them at greater risk for exposure to the virus.

In introductory remarks Thursday, the FDA's Peter Marks urged the independent panel to "harmonize" its decision with what has already been decided for Pfizer in order to avoid unnecessary confusion for the public. 

Pfizer boosters have been authorized for people over 65, all adults with underlying medical conditions and adults whose job puts them at high risk. 

The vote is scheduled for about 4:45 p.m.

-ABC News' Sasha Pezenik

Initial distribution of the Pfizer vaccine for children ages 5 to 11 years old will be based on a state’s population of eligible children, according to a new planning document distributed to state immunization managers by the U.S. Centers for Disease Control and Prevention.

The document, obtained by ABC News, was sent to state officials in advance of the vaccine being authorized by federal regulators. The authorization could happen as soon as early November.

The pediatric vaccine will be shipped in 100-dose packs, each with 10 vials, the document said. 

A person familiar with the planned rollout told ABC News that while the government purchased 65 million doses total, the initial shipment may be closer to the 10 million to 20 million range. 

After an initial distribution, a "weekly supply will be made available to help sustain the network," according to the document.

-ABC News' Sasha Pezenik