Moderna Inc. /zigman2/quotes/205619834/composite MRNA +12.20% announced Tuesday afternoon that it has reached an agreement to sell 100 million doses of its investigational vaccine for COVID-19 to the U.S. government for more than $1.5 billion. "Breaking: Moderna announces a supply agreement with the U.S. government to secure an initial 100 million doses of mRNA-1273 for up to $1.525 billion with option granted to U.S. government to purchase up to an additional 400 million doses," the company revealed in a tweet . Moderna shares jumped more than 8% in after-hours trading immediately following the announcement. Earlier studies of Moderna's vaccine candidate produced antibodies for COVID-19, the company previously announced, and it is currently operating a Phase 3 study of the vaccine. "The U.S. government has announced that consistent with its commitment to free access to COVID-19 vaccines, Americans will receive mRNA-1273 at no cost for the vaccine itself," Moderna added in a subsequent news release . "As is customary with government-purchased vaccines, healthcare professionals could charge for the cost of administering the vaccine." Moderna shares have exploded higher amid the frenzy to develop a COVID-19 vaccine during a worldwide pandemic, with the share price jumping more than 250% so far this year as the S&P 500 index /zigman2/quotes/210599714/realtime SPX +0.36% has gained 5.2%.