On a call with reporters, Gen. Gustave F. Perna, the chief operating officer for Operation Warp Speed, the federal effort to speed a vaccine to market, said that 2.9 million doses would be sent out once the vaccine is authorized, and 2.9 million saved for booster shots, which are given three weeks later. The remaining 500,000 of an available 6.4 million doses are to be held in reserve, in case they are unexpectedly needed.

Health care workers and nursing home residents will be the first groups to receive the vaccine.

General Perna said he had decided to set aside the doses earmarked for the second shot out of caution.

“Eventually, we will become much more confident in our manufacturing, our distribution process, state handling, et cetera,” he said. “And then the requirement for reserve won’t be necessary.”