It’s still unknown when a COVID-19 vaccine might be available in the United States. But when one is first approved, there may only be 10 million to 15 million doses available, which may be enough to cover around 3% to 5% of the U.S. population. That’s according to estimates from Operation Warp Speed, the government’s vaccine project, published in a draft framework from the National Academies of Sciences, Engineering and Medicine.