Moderna Inc said on Wednesday the US government has bought another 200 million doses of its authorized COVID-19 shot, including the option to purchase other coronavirus vaccine candidates from the company's pipeline.

The United States has now ordered a total of 500 million Moderna vaccine doses to date, with 110 million set for delivery in the fourth quarter and 90 million to be delivered in the first quarter of 2022.

Moderna, which has supplied 217 million doses of its shot to the US government as of Monday, said the additional doses were bought to ensure continuous supply through the first quarter of next year.

More than 129 million doses of Moderna's vaccine have so far been administered in the United States, according to the latest data from the Centers for Disease Control and Prevention.