The Biden administration announced Tuesday that it is working to purchase an additional 200 million doses of the two coronavirus vaccines that have been authorized for emergency use in the United States. The administration hopes to have enough vaccine supply for almost the entire U.S. population by the end of summer. The purchase of 100 million doses each of Moderna and Pfizer’s vaccines would grow the available supply by 50 percent to a total of 600 million doses by summer — enough to fully vaccinate 300 million people as both vaccinations require two doses. A senior administration official reportedly told the Washington Post that the additional doses would be delivered this summer. President Biden on Monday increased his administration’s vaccine distribution goal to 1.5 million shots administered per day, up from his earlier goal of 1 million shots per day. Health care workers were already administering more than a million shots as of Wednesday. “I feel confident that by summer we’re going to be well on our way to heading toward herd immunity,” Biden said. The increased target marked a shift from Biden’s remarks on Thursday, when the president challenged a reporter who asked if the vaccination goal was “high enough,” since “that’s basically where the U.S. is right now.” “When I announced it you all said it wasn’t possible. Come on, give me a break, man,” Biden said then. If the U.S. can administer 1.5 million shots in 100 days, around 75 million people could be fully vaccinated, or 23 percent of the country’s population. The vaccine rollout has faced two significant obstacles, an official told the Washington Post: supply of vaccines and states and localities’ ability to quickly administer vaccines. The federal government has distributed roughly 20 million more doses than have been administered to people, according to the Centers for Disease Control and Preventions. The Biden administration says it will boost supply to states, tribes and territories over the next three weeks, from a current 8.6 million doses a week to a minimum of 10 million doses per week. It also plans to provide allocation estimates three-weeks out, instead of the one-week look-ahead that states have been receiving. “This increased transparency will give state and local leaders greater certainty around supply so that they can plan their vaccination efforts and administer vaccines effectively and efficiently,” the White House said.