The US said it was purchasing 100 million more doses of the Covid-19 vaccine candidate developed by Moderna, amid reports the government passed on the opportunity to secure more supply of the Pfizer jab, reports AFP.

The agreement brings the total number of Moderna doses bought by the US to 200 million, enough to immunise 100 million people with the two-shot regimen.

"Securing another 100 million doses from Moderna by June 2021 further expands our supply of doses across the Operation Warp Speed portfolio of vaccines," said Health and Human Services Secretary Alex Azar.

A statement by the New York state-based biotech firm said that the new order would be delivered by the second quarter of 2021, while the first order would be completed by the first quarter.