The U.S. government has agreed to purchase an additional 100 million doses of Pfizer (PFE) and BioNTech’s (BNTX) vaccine in 2021, ramping up the available supply as the world’s largest economy continues to grapple with a deadly surge in COVID-19 infections that has flooded hospitals around the country.

The agreement will bring the total number of doses to 200 million, and will cost $4 billion. Pfizer had previously noted the U.S. government refused to purchase additional doses after several offers in the summer, leading the company to commit to doses for other countries. Though the agreement with the company, signed in July, allows for an additional 500 million doses, Pfizer was unable to commit to a timeline for the government to sign.

At the time, Health and Human Services Secretary Alex Azar noted the government had committed to about 600 million doses— or enough to vaccinate most Americans with the required two doses— from several companies.

The agreement Wednesday leaves an additional 400 million on the table for future purchase from Pfizer. HHS also recently announced an additional 100 million doses purchased from Moderna (MRNA) prior to its emergency use authorization (EUA) last week.

Both agreements target all doses to be delivered by summer 2021, when the government hopes to have a majority of Americans vaccinated. Yet some health experts, like National Institute of Allergy and Infectious Diseases director Anthony Fauci, have said that timeline is optimistic, and will likely end up falling into the third quarter of 2021.

UK strain might already be here

Meanwhile, both Pfizer and Moderna have said they are studying their vaccine’s effectiveness against a new strain of the virus that is spreading quickly in the U.K. On Wednesday, the country reported nearly 40,000 new cases, the highest since the pandemic began.

The strain is said to be far more transmissible, and has already showed up in other countries, spurring travel bans around the world. The U.S. Centers for Disease Control and Preparedness has said the strain has likely already made it to the U.S.; however, because the country isn’t sequencing its samples as quickly as the U.K. and other countries, it’s unclear how prevalent the strain may be.

“Viruses have only been sequenced from about 51,000 of the 17 million US cases,” the CDC said.

The U.S., still leading the world in cases and deaths, is not banning travel from the U.K. at the moment, which increases the potential of the strain’s spread.

“Ongoing travel between the United Kingdom and the United States, as well as the high prevalence of this variant among current U.K. infections, increase the likelihood of importation. Given the small fraction of US infections that have been sequenced, the variant could already be in the United States without having been detected,” the CDC said.

Experts have said that while the strain caused initial concern because the changes occurred in the spike protein— the target of many vaccines authorized and being developed— it is likely to be weaker and still respond to vaccines.

More from Anjalee:

• COVID-19 peak will likely hit US in mid-January: Experts

• What you need to know about at-home, over-the-counter COVID-19 tests

• Biden COVID-19 adviser: Vaccine news is great, but we still have a ways to go

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