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Two Covid-19 vaccines are expected to be ready by the end of the month, and already one thing is clear: There won’t be enough vaccines to go around.

Tough decisions will have to be made. Who gets the vaccine first? Who gets it next? Who will have to wait months?

Those historic choices will be left to the Advisory Committee on Immunization Practices, a panel of independent advisers to the US Centers for Disease Control and Prevention. They’ll meet Tuesday afternoon to vote on who gets the vaccine first.

“This is an unprecedented vote that ACIP will be making,” said Dr. William Schaffner, a member of the panel since 1982.

Members of the group have been deliberating for months, consulting with bioethicists and other experts.

“The discussions have been elaborate and consultations broad. I can assure you discussions among all my colleagues have been very trenchant,” Schaffner said.

Vaccinations are expected to begin in mid- to late December. The panel is set Tuesday to vote on whether the first group to be vaccinated should be health care workers and residents of nursing homes.

If those two groups are first – or “Phase 1a” in CDC parlance – then other high priority groups will have to wait, including people with underlying medical conditions, essential workers such as police officers and firefighters, and elderly people who are not in nursing homes.

Those who don’t fit into any of these categories will have to wait even longer, likely until the spring.

Vaccines by the numbers

So far, two pharmaceutical companies, Pfizer and Moderna, have applied to the US Food and Drug Administration for authorization for their Covid-19 vaccines.

Gen. Gustave Perna, chief operating officer of Operation Warp Speed, said last week that he anticipates 6.4 million doses could be available in mid-December, and then more doses available in the coming weeks.

“I do feel confident that we’re going to get to as close as we can to 40 million at the end of the year,” he said.

But hundreds of millions of doses – not tens of millions of doses – are needed for just the high priority groups.

In the United States, there are some 3 million people living in nursing homes and other long-term care facilities; about 21 million health care workers; about 53 million senior citizens; some 87 million essential workers; and more than 100 million people with medical conditions that put them at high risk for complications of Covid-19, such as diabetes and severe obesity, according to an ACIP report.

That adds up to about 261 million people. The Pfizer and Moderna vaccines require two doses several weeks apart, so eventually these high priority groups will need more than 500 million doses.

Making an historic decision

Schaffner said he can think of only one other time – the H1N1 flu pandemic in 2009 – that ACIP has had to set priority groups because there initially wasn’t enough vaccine to go around.

During the current pandemic, the panel’s working group has met weekly to deliberate on the various options.

“The committee is trying very hard to be transparent and thorough and thoughtful so that we can provide the greatest good for the greatest number,” he said.

ACIP has 14 voting members – Schaffner, as a liaison member is not one of them – including pediatricians, infectious disease experts, and a lawyer.

The panel will be guided by several questions, according to a presentation last week by the ACIP COVID-19 Vaccines Work Group. For example, which group has already suffered the biggest burden from Covid-19? Which group is at the highest risk of becoming infected? Which group would most benefit from the vaccine?

The group has already received guidance from a National Academy of Sciences panel, which in September issued a report saying first responders and high-risk workers in health care facilities should be first to receive the vaccine.

Vaccination timeline

ACIP is voting on the prioritization Tuesday because by Friday, states need to submit their orders for vaccines, according to a CDC spokesperson.

Friday is the “locked and loaded” date, said Claire Hannan, executive director of the Association of Immunization Managers, which is helping states with their immunization plans.

“These things have to be checked off and in place December 4,” she said.

Dr. Anthony Fauci said last week that he expects vaccinations to begin toward the latter part of December “very likely some time a bit before the Christmas holidays.”

Vaccinations for people who do not fall into high-risk groups will likely start at the end of April and continue “going into May, June, July,” said Fauci, the director of the National Institute of Allergy and Infectious Diseases.

Another federal official said it wouldn’t take that long.

Asked about his expectations for how many Americans will be vaccinated against Covid-19 by June, Paul Ostrowski, director of supply, production and distribution for Operation Warp Speed, told MSNBC that “a hundred percent of Americans that want the vaccine will have had the vaccine by that point in time.”

Next steps

Both the Pfizer and the Moderna vaccines have shown about 95% efficacy, a stunning number beyond almost anyone’s expectations.

Before they’re put on the market, the vaccines need to get authorization or approval from the US Food and Drug Administration. An independent panel of experts that advises the FDA is scheduled to review Pfizer’s application on December 10 and Moderna’s application on December 17.

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The FDA will then take “days to weeks” to decide whether to issue emergency use authorization to the vaccines, according to Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research.

Then, it’s ACIP’s turn.

Even though the committee is voting Tuesday, it will meet and vote again a day or two after the FDA issues its authorization, according to the CDC spokesperson.

States do not have to abide by ACIP’s recommendations, and instead can come up with their own frameworks for vaccine prioritization, according to the CDC spokesperson. However, historically, states have abided by ACIP’s recommendations.

“I don’t see states deviating from the ACIP guidance,” Hannan said.

CNN’s John Bonifield and Sierra Jenkins contributed to this report.