Pfizer Inc and German partner BioNTech said on Friday the U.S. government had purchased 200 million additional doses of their COVID-19 vaccine and had the option to buy an updated version of the vaccine targeting new variants of the virus.

The deal comes as the Delta variant of the coronavirus sweeps the country and drives up infections, contributing to the debate over whether or not Americans will need a booster dose this fall.

It also follows the government's move in June to buy 200 million more doses of Moderna Inc's COVID-19 vaccine.

The announcement brings the total number of doses of the Pfizer/BioNTech vaccine to be supplied to the United States to 500 million, out of which roughly 208 million doses have already been delivered, as of Thursday's data from the government.

"These additional doses will help the U.S. government ensure broad vaccine access into next year," Pfizer Chief Executive Officer Albert Bourla said in a statement.

 
Pfizer last year signed a deal with the U.S. government for 100 million doses of the vaccine for nearly $2 billion, with an option to buy 500 million more doses.

A majority of the new doses will be supplied by the end of the year, and the remaining 90 million will be delivered by April 30, the companies said.

Pfizer earlier this month said the companies plan to seek authorization from U.S. and European regulators for a booster dose of their COVID-19 vaccine.

The U.S. government has said Americans who have been fully vaccinated do not need a booster COVID-19 shot at this time.

The companies have designed a new version of the vaccine targeting the Delta variant, which they plan to test in the coming weeks, but have said the current vaccine could also provide protection against the variant.