The US Food and Drug Administration on Friday authorized boosters of the Pfizer/BioNTech and Moderna Covid-19 vaccines for all adults.

The agency expanded emergency use authorization for booster doses of both the mRNA vaccines beyond who was previously eligible; boosters had been authorized for anyone 65 and older who was vaccinated with the Pfizer or Moderna vaccines at least six months ago and for certain adults at high risk of infection or of severe disease.

Pfizer and BioNTech requested authorization last week based on results of a Phase 3 trial involving more than 10,000 participants; it found boosters were safe and had an efficacy of 95% against symptomatic Covid-19 compared with the two-dose vaccine schedule in the period when the highly transmissible Delta was the dominant strain. Pfizer released the booster efficacy data last month; it has not yet been peer-reviewed or published.

Moderna requested authorization of its 50-microgram booster dose for all adults on Wednesday. The company said the FDA based the EUA on the "totality of scientific evidence shared by the company," including data that showed neutralizing antibodies had waned at about six months.

Recent real-world studies suggested that immunity from Covid-19 vaccines may begin to wane and protection against milder and asymptomatic disease, in particular, may drop. Studies have shown that booster doses restore that immunity.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN that recent data from Israel show that, among people age 60 and older, those who received a booster were less likely to become severely ill than vaccinated people who had not received a booster. Rates of severe disease remained highest among those who weren't vaccinated.

"Throughout the course of the COVID-19 pandemic, the FDA has worked to make timely public health decisions as the pandemic evolves. COVID-19 vaccines have proven to be the best and highly effective defense against COVID-19. Authorizing the use of a single booster dose of either the Moderna or Pfizer-BioNTech COVID-19 vaccine for individuals 18 years of age and older helps to provide continued protection against COVID-19, including the serious consequences that can occur, such as hospitalization and death," Acting FDA Commissioner Dr. Janet Woodcock said in a statement.

The US Centers for Disease Control and Prevention's vaccine advisory committee will meet Friday to discuss expanding booster eligibility. Boosters could officially be administered to all adults after CDC Director Dr. Rochelle Walensky signs off on a recommendation.

However, the majority of adults are already eligible to receive boosters, and several states have officially opened up boosters to all adults already.

'Everyone is eligible'

Expanding booster eligibility to all adults in the United States may not change the logistics around getting shots into arms.

The vaccine supply for boosters is already in place in many places. There are no "extra steps" on the administration side of the booster rollout that need to happen, Adriane Casalotti, chief of government and public affairs at the National Association of County and City Health Officials, told CNN.

"Looking national, we have enough vaccine. There are lots of places that have appointments. Obviously, that varies a little bit depending upon where you live geographically because there's more demand in some areas than others," Casalotti told CNN.

"But that being said, I think the main preparation is on the communication side," Casalotti added. "The communication for the local health care providers and public health departments is that now you don't have to worry about eligibility if someone asks if they need to get a booster. This is the information -- everyone is eligible after six months."

More than 32 million people in the United States -- about 16% of those who are fully vaccinated -- have received a booster dose of Covid-19 vaccine, according to CDC data.

About 18 million seniors have received a booster dose of Covid-19 vaccine, according to CDC data, accounting for more than half of all booster doses administered and increasing the immune response for more than a third of fully vaccinated seniors.

However, more than twice as many seniors -- about 40 million people age 65 or older -- completed their initial vaccine series at least six months ago. Less than half have complied with current CDC guidance about who should get a booster.

What's next?

CDC's vaccine advisers' meeting starts at noon on Friday, and signoff could come from Walensky any time after the advisers vote.

The last time CDC's vaccine advisers met to discuss Covid-19 booster shots was in October.

"We're looking to see, compared to a month or so ago when the discussion was first had, what have we seen as far as the waning of immunity, what have we seen in terms of Covid rates,"said Dr. Marci Drees, chief infection prevention officer and hospital epidemiologist for Delaware-based ChristianaCare, told CNN on Thursday.

Drees is a Society for Healthcare Epidemiology of America liaison to the CDC's Advisory Committee on Immunization Practices.

The US is now averaging 94,943 new Covid-19 cases each day, according to Johns Hopkins University -- a 31% increase over last week and back to levels last seen more than a month ago. Midwestern states account for more than a third (38%) of new cases.

There's concern that winter weather that drives people indoors and holiday gatherings could lead to even more cases.

"So, we certainly want people to be as well protected as they could possibly be going into the season," Drees said. "I think we're at the point now where for lots of people who were vaccinated last winter and spring, now would be the time that their protection is waning."Once people do receive their Covid-19 vaccine booster shots, it's not clear when they might need another vaccine dose -- if ever.

"That certainly is the million-dollar question. We know that the boosters boost people's immunity back up to that 90% to 95% range in the short term. We don't know how long that will last," Drees said.

"In some ways, we're forging a new path -- and a lot of it is driven by can we get enough immunity in the population so that we can really shut down further transmission," Drees said. "We know that boosters are not going to end the pandemic. They will help and prevent people from getting sick, but we really still need to push on getting first and second doses into people as well."