The first vaccination sites are expected to receive coronavirus vaccines today, the next step of an extraordinary endeavor that's brought us to the beginning of the end of the pandemic.

Yes, but: The virus continues to devastate communities across the country at record levels, causing some experts to question the ethics of how the government is distributing the first vaccines.

What's happening: The initial batch of doses available to Americans is extremely limited, and each recipient must get two shots.

• That means that of of the 6.4 million available doses of Pfizer's vaccine, roughly half are being sent out at first — 2.9 million, per the NYT. The other half will be distributed in a few weeks, to ensure that recipients receive their second shot 21 days after the first.

Between the lines: Some experts are questioning whether any doses should be held back given how acute the crisis is, and how many vulnerable people's lives may be saved by receiving even one dose of the vaccine.

• Pfizer has promised 20 million doseswill be available by the end of the year, making the decision to hold some of the initial doses back even more ethically fraught, the experts say.
• "What I’m asking is whether it is ethical to hold back those doses to be 100% sure the 2nd shot happens exactly in 21 days, or whether it is better to not hold back those doses, vaccinate twice as many people, and take a small risk that the Pfizer doses as promised don’t arrive as planned by end of year and some people are delayed in that 2nd dose," said Walid Gellad, a professor of medicine at the University of Pittsburgh.
• "We know the first dose has an effect, and we are at the peak of this pandemic," he added.

Details: Pfizer's clinical trials results have indicated that even one dose of its vaccine confers some degree of protection upon the recipient.

• "We know the first dose is partially protective," former FDA Commissioner Scott Gottlieb told CNBC's Squawk Box. "You want to try to push out as many doses as possible to give as many people as possible some benefit."
• "There is some wiggle room in terms of when they receive that second dose," Gottlieb added.

The other side: "The reason why we're holding on to the second dose as well as some reserve is that we don't have absolute confidence in the cadence, not because Pfizer or Moderna or the supporting manufacturers...aren't diligent in their process, but it is such a delicate process, we want to ensure perfection in the vaccine," Army Gen. Gustave Perna, who is with Operation Warp Speed,told reporters on Saturday.

• As time goes on, the need to hold back doses will diminish, Perna said. "Holding the second dose is just until we have ultimate confidence and we've built up stocks to ensure that we can get the American people their second dose."

The big picture: There's a larger scientific conversation around how to handle the clinical trial results. Some scientistshave warned emphatically that giving only one dose to people is a bad idea, even if it'd double the number of people who could be vaccinated in the short-term.

• "It will be an absolute disaster if a large number of people only take one dose of the vaccine. It is very likely that protective immunity will wane rapidly in individuals who only take the first shot and efficacy will be nowhere near the 95% reported after the two-dose regimen," tweeted Carlos del Rio, executive associate dean for Emory School of Medicine & Grady Health System..
• "Too much uncertainty in short- and long-term efficacy," agreed Natalie Dean, a professor specializing in vaccine study design at the University of Florida. "And the optics of changing the plan now, for a regimen that hasn’t been tested directly, are not great."

But some argue that we should at least study a one-dose regimen.

• "*IF* a single dose DOES work in less vulnerable [people] and IS sufficiently durable (even if not perfect), it might mean vaccinating billions of additional people in 2021 & reach herd immunity faster," tweeted Michael Mina, a professor of epidemiology at Harvard University.