(Reuters) - Moderna Inc said on Monday it would produce at least 600 million doses of its COVID-19 vaccine in 2021, up by 100 million doses from its previous forecast, as the United States continues to roll-out the vaccine. The company was working to invest and hire in order to deliver up to 1 billion doses this year, the higher end of its production forecast, it said. Moderna said it has so far supplied about 18 million doses to U.S. government as part of a deal for 200 million doses. It has also signed a deal with the Canadian government for 40 million doses. (This story refiles to add dropped word in paragraph two)