The Food and Drug Administration has not yet given emergency authorization or approval to any of the coronavirus vaccines now in development. Vaccines from Moderna, Pfizer and AstraZeneca are now in the final stage of clinical trials in the United States, and the government has pre-purchased hundreds of millions of doses of at least a half-dozen experimental shots.

Asked about comments from the government's top infectious disease expert, Anthony Fauci, and CDC Director Robert Redfield about vaccines being broadly available mid- to late next year, Trump said, "We think we can beat that number very substantially."

But it is still unclear which of those available doses will be usable. Experts say it's unlikely that all the vaccines now in development will prove safe and effective.

AstraZeneca recently paused its end-stage, Phase III trials after a woman who received the shot developed serious side effects. It is still not clear whether her spinal inflammation was related to the shot itself. Trials outside the U.S. have resumed, but the American trial is still on hold.

Moderna has said it can produce 20 million doses by the end of the year. Pfizer has promised 100 million shots worldwide in the same timeframe, but has not disclosed how many would go to the U.S. Executives from both companies have suggested they could have data for emergency use authorization as early as October, although clinical trial blueprints released yesterday by the two firms suggest the first trial data won't be available then.

CDC's Advisory Committee on Immunization Practices in August projected that the U.S. would have anywhere from 40 million to 45 million doses of the Moderna and Pfizer vaccines, combined, by late December 2020.

Because the vaccines are two-dose regimens, 40 million doses would be enough to vaccinate 20 million people. It is not clear whether Trump's promise to have enough vaccine for all 330 million Americans by April means that the government will have enough vaccine to give each person the first of two shots, or the full two-shot vaccination.

Other companies receiving U.S. funding including Johnson & Johnson are expected to enter Phase III trials soon.

While manufacturers typically wait until their drug is approved to begin mass production, several companies are scaling up manufacturing with government backing so their vaccines — if approved — can swiftly be distributed.

The plan is for shots to be distributed within 24 hours of emergency authorization or approval, Paul Mango, HHS’ deputy chief of staff for policy, said on a call with reporters on Wednesday. The health department issued a distribution playbook for states the same day. Trump repeated the promise in the Friday press briefing.

Mango in the call laid out an optimistic timeline for vaccines to be ready in the fourth quarter for high priority populations such as health care workers and the elderly. “As supply equals demand and exceeds demand into 2021, our distribution principles will change,” he said.

Other senior health officials including Redfield and HHS testing czar Brett Giroir have said widespread vaccine distribution is more likely in the middle of next year.

Meanwhile Food and Drug Administration Commissioner Stephen Hahn said last week that the agency would soon release new guidance on how a Covid-19 vaccine could be authorized for emergency use. The guidance is expected to raise the requirements for authorization, which is still a bar lower than full approval.

Zachary Brennan contributed to this report.

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