There’s a series of fairly straightforward, if intricate, regulatory steps by the FDA and the Centers for Disease Control (CDC) which will determine the answer. Here’s a timeline of what to keep an eye on in the coming weeks and months.

The FDA COVID vaccine process

Pfizer and partner BioNTech officially filed for FDA emergency use authorization (EUA) of their experimental COVID vaccine, which they say is about 95% effective, on November 20. U.S. biotech Moderna announced similarly encouraging results for its own candidate last week, suggesting the company will submit its own EUA at the end of November or in early December.

Emergency authorizations have a lower threshold for a regulatory green light than a full approval, which is a more time-consuming and involved process. Soon after Pfizer got the ball rolling, the FDA announced that its Vaccines and Related Biological Products Advisory Committee (VRBPAC) would convene on December 10 to discuss Pfizer’s data and make recommendations regarding the vaccine.

Advisory committees are made of a group of experts who counsel the FDA on a therapy. Their recommendations aren’t binding, but the agency has historically followed such committees’ lead. It’s possible that the VRBPAC could vote to recommend approval, or to recommend that the Pfizer COVID vaccine is indeed safe and effective, on December 10.

Then, in a separate decision, the EUA would head to the full FDA for consideration. That process could take another few days or about a week, meaning the vaccine could theoretically be granted its authorization by December 18 or earlier.

At this point, the vaccine would be ready for distribution. But just where that distribution may occur will likely be an ad hoc process. Companies across the supply chain such as drug distributors, hospitals, or local pharmacies may receive the initial doses, though it remains unclear. The Department of Health and Human Services (HHS) has said it’s begun testing various distribution networks, including centers that are set up by states and locales to get the vaccine.

But how soon Americans can actually get it ultimately depends on the CDC.

Who decides the first people to get a COVID vaccine

The FDA is limited to a dual mandate: Judge a drug candidate by its safety and efficacy profile. It can’t speak to issues such as pricing and in the case of a COVID vaccine, the CDC will have to convene its Advisory Committee on Immunization Practices (ACIP) to determine how to prioritize in the face of what’s expected to be enormous demand. The group released an ethical framework that will serve the basis of that conversation this week.

That meeting could theoretically happen right after the FDA grants a COVID vaccine emergency authorization, so perhaps sometime during the week of December 14 (although there’s usually more of a lag between an authorization and such a meeting). Once the ACIP has voted on its distribution and prioritization guidance, CDC director Robert Redfield would give it a final blessing.

At that point people who meet the guidance criteria could begin getting immunizations. But there’s a pandemic-related twist: On Tuesday, HHS Secretary Alex Azar said that state governors would have the final say in how they decide to prioritize vaccine distribution regardless of the ACIP’s recommendations. While this is meant to provide states with different demographics some flexibility, it may also lead to hugely divergent distribution strategies, much like the early days of COVID testing.

It’s widely expected that the first tranche of Americans to have access to the Pfizer vaccine are those at highest risk of coronavirus infection, such as the elderly, certain minority groups, health care workers, and people with underlying health conditions such as diabetes and heart disease.

But, if everything works out for Pfizer, certain people may be able to start getting their COVID shots by mid-December, most likely at state-run locations or through companies like CVS Health which have teamed up with the government at first. State health professionals will have to determine their final guidance for who is prioritized for a vaccine (again, much like distribution of COVID tests) and then local health care sites, retail, state-run, or otherwise, with the capacity to implement them will follow that guidance to distribute doses.

It’s a massive undertaking which will likely be fluid as states, governments, and medical facilities figure out which strategies work best. “If all goes well, we could be distributing vaccine soon after Dec. 10,” Azar said on Tuesday.

“We believe we can distribute vaccine to all 64 jurisdictions within 24 hours of FDA authorization. Then we hope administration can begin as soon as the product arrives.” He also said that firms such as CVS, in collaboration with the federal government, could begin innoculating vulnerable populations in nursing homes within 48 hours of FDA authorization.

Widespread distribution will take more time

That doesn’t mean just anyone will be able to get Pfizer’s vaccine by next month. First, there’s the manufacturing problem. Creation of the actual vaccine will have to ramp up considerably in the coming months given that somewhere around 40 million doses are expected to be available by the end of the year.

But both Pfizer’s and Moderna’s experimental vaccines require two separate doses administered several weeks apart. That cuts the number of patients who might get one by then in half.

Companies working on these COVID drugs have said they plan to get up to hundreds of millions of doses, which will have to distributed not just in the United States but across the globe, by the end of 2021. For Americans generally, public health experts believe widespread use could begin by the spring or early summer of next year.