Shares of Moderna Inc. /zigman2/quotes/205619834/composite MRNA -10.62% were down 2.1% in premarket trading on Wednesday after the company said it was in talks with the U.S. government to supply an additional 100 million doses of its COVID-19 vaccine. The delivery, which would occur in the third quarter of 2021, would bolster the U.S. government's total order from Moderna to 300 million doses. Moderna's vaccine requires two doses so that means the company could be supplying doses for 150 million people. The Biden administration said Tuesday that it also planned on ordering an additional 100 million doses from BioNTech SE /zigman2/quotes/214419716/composite BNTX -7.36% and Pfizer Inc. /zigman2/quotes/202877789/composite PFE +2.63% . "We expect these additional 200 million doses to be delivered this summer," President Joe Biden said Tuesday. "This is enough vaccine to fully vaccinate 300 Americans by the end of the summer, beginning of the fall." Moderna's stock has soared 568.1% over the past 12 months. The S&P 500 /zigman2/quotes/210599714/realtime SPX -1.31% is up 16.8.%.