Senior Biden administration officials said Tuesday that the federal government will purchase 200 million more vaccine doses from Pfizer and Moderna as part of a “National Action Strategy” to combat the COVID-19 pandemic. President Joe Biden has promised to deliver 100 million doses in his first 100 days as president. The administration also pledged to vaccinate the majority of Americans by the latter half of this year. “With these additional doses, the U.S. will have enough vaccine to fully vaccinate 300 million Americans by the end of this summer,” the officials said. The additional doses purchased will raise the weekly supply allotted to states from 8.6 million to 10 million, and the administration promised to give states a forecast of vaccine supply three weeks in advance rather than one week, as is the current norm.