Within seven days of taking over as US president, Joe Biden on Tuesday declared that his government is "working to purchase" a total of 200 million additional doses of two authorised vaccines from Pfizer and Moderna which will be enough to vaccinate 300 million Americans by late summer this year.

"This increases the total vaccine order for the US by 50 per cent, from 400 million to 600 million with these additional doses expected to deliver this summer. With these additional doses, the US will have enough vaccine to fully vaccinate 300 million Americans by the end of this summer," Biden said Tuesday evening. Summer in the US begins June 20.