Moderna Inc. said the U.S. government exercised its option to buy 100 million additional doses of the company’s coronavirus vaccine candidate, doubling the amount of the shot it initially committed to purchase and easing concern that the U.S. could be left with a vaccine shortfall.

Moderna said Friday that the order, worth US$1.65 billion, would be delivered during the second quarter of 2021. The Cambridge, Massachusetts-based drugmaker will be able to provide a continuous supply of vaccine doses through the end of June, according to the U.S. government.

Worry emerged this week that the U.S. might not have sufficient supply of COVID-19 vaccines following reports that the government had passed on an offer from Pfizer Inc. for more of the shot it developed with BioNTech SE, which is expected to gain authorization from the Food and Drug Administration.

U.S. officials have said they have confidence the government will be able to meet nationwide demand, having secured supply agreements for six different experimental candidates.

As part of Operation Warp Speed, the U.S. initiative to rapidly develop and secure vaccines for COVID-19, the U.S. has the option to buy another 300 million doses from Moderna at a fixed price of US$16.50 per shot, the company said. The experimental messenger RNA vaccine, which is administered in two doses, is being evaluated by the FDA, and could receive an emergency-use authorization before the end of the month.

“This new federal purchase can give Americans even greater confidence we will have enough supply to vaccinate all Americans who want it by the second quarter of 2021,” Health and Human Services Secretary Alex Azar said in the statement.

Moderna shares gained 2.6 per cent in late trading in New York. So far this year, the stock has surged in value by more than eightfold.

Moderna has said it expects to produce 500 million to 1 billion total doses of its vaccine in 2021. It is currently making U.S. supply at its plant in Massachusetts and another factory in New Hampshire owned by manufacturing partner Lonza Group AG. Lonza Group is also producing supply for other countries at a plant in Switzerland.

The company has agreed to supply shots to the European Union, the U.K., Japan, Canada, Switzerland, Israel and Qatar, among others.

HHS and the Department of Defense have provided US$3.2 billion to expand the manufacturing capacity reserved for Moderna’s two-dose vaccine regimen, known as mRNA-1273.