Moderna Inc said that the US government has bought another 200 million doses of its authorised Covid-19 shot, including the option to purchase other coronavirus vaccine candidates from the company's pipeline, Reuters reports.

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

Moderna, which has supplied 217m 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.