People in the US could now get the new subvariant booster 2 months after their last shot of the COVID-19 vaccine, with Moderna's new shots authorized for Americans as young as 18, and Pfizer and BioNTech can be given starting from age 12.

According to Dr. Robert Califf, the FDA commissioner, "As we head into fall and begin to spend more time indoors, we strongly encourage anyone who is eligible to consider receiving a booster dose with a bivalent COVID-19 vaccine to provide better protection against currently circulating variants" - meaning that deliveries of the new doses from Pfizer-BioNTech and Moderna can now begin as multiple states and local health departments have already placed pre-orders last month for the first set of shipments.

Per its spokesperson, Pfizer can ship up to 15 million doses of the new booster by the end of next week, pending "final quality control checks." However, similar to previous COVID-19 vaccine rollouts, vaccine providers will need to wait for the Centers for Disease Control and Prevention's (CDC) own recommendation before administering their first shots, expected by the end of this week, following a two-day meeting of the agency's outside vaccine advisors. 

The new rollouts mark the first significant change to the COVID-19 vaccines since they were provided earlier in the pandemic and Pfizer intends to submit a request to the FDA "in early October" for giving the new booster to children starting at the age of 5 years old and is also working on a submission for boosting kids as young as 6 months old.

By mixing a component from their original "prototype" shots with another, the vials of the so-called "bivalent" shots from each company are intended to target the Omicron subvariants BA.4 and BA.5 - now hitting hard across the US.

Pfizer's CEO Albert Bourla disclosed, "As we head into the fall and winter season, with the potential for greater SARS-CoV-2 spread in schools and at work, it is important to stay up to date with vaccines as a first line of defense against COVID-19 illness."

Earlier this year, scientists are warning that the wide and fast-spreading Omicron is an indicator that it will not be Covid-19's last variant.

Why are the booster shots being changed?

The decision was the result of meetings of the FDA's vaccine advisors earlier this year to weigh the issue, which at the time, a majority of the committee leaned toward an approach close to that of Pfizer and BioNTech to expedite the authorization of new shots that might boost their effectiveness ahead of another feared deadly winter wave of the virus.

Although the FDA said it would not require lengthy clinical trials of the Omicron shots before authorization, it would rely largely on trial data collected from testing the immune response triggered by the new shots in animals, as well as data from other formulas trialed by the vaccine makers.

Dr. Califf added on the Public Health On Call podcast on Tuesday, "In the midst of a pandemic, if you wait for all that data to come in, you've missed the boat. And so you have to be preemptive," comparing the authorization to updating the annual flu vaccine, citing "the totality of the evidence" supporting their decision.

CDC Director Dr. Rochelle Walensky indicated her approval of signing off on the new booster shots and pointed to her agency's calculations, which show the BA.4 and BA.5 subvariants making up virtually all new infections across the US right now.

"The strategy now is to tailor vaccine for giving us the largest breadth of response, ideally one that would have less waning over time," Walensky told the Conversations on Health Care radio program.

But even with the presence of the approval, the two vaccine makers disclosed plans to complete human clinical trials of these new shots, with federal health officials asking for these trials to track the vaccine's ability to curb future virus variants later this year, which might look very different from the strains circulating now. 

Amid annual flu shots and a current monkeypox vaccine rollout, twenty million doses were made available for vaccine providers to pre-order, from states to pharmacies, according to a spokesperson for the Department of Health and Human Services - all 50 states and the District of Columbia have requested shots.

According to CDC statistics, the US is currently averaging less than 40,000 first doses a day and the CDC last month urged state and local health departments to prepare for distributing the early shipments to providers to serve "those at highest risk for severe COVID-19 disease" first, like in nursing homes. They also announced that not all providers will continue to have stock of the primary series COVID-19 vaccines.