US government is buying another 100 million doses of Moderna's COVID-19 vaccine, bringing the total to 200 million

• The US government has ordered another 100 million doses of Moderna's coronavirus vaccine, bringing the total to 200 million, the company announced Friday.
• If the vaccine receives emergency-use authorization from the FDA, Moderna plans to deliver 20 million doses by the end of the month, then the rest over the first six months of 2021.
• The US has the option to purchase another 300 million doses from Moderna.

The US government has agreed to purchase another 100 million doses of Moderna's coronavirus vaccine, the company announced on Friday.

That's on top of a $1.5 billion order placed in August for 100 million doses, bringing the total to 200 million, according to a press release from Moderna. Since the vaccine is administered in two shots, that total is enough for 100 million people.

All in all, Operation Warp Speed - the US government's coronavirus vaccine program - has given Moderna $4.1 billion for its vaccine, including $955 million to advance its clinical trials.

The government has also purchased 100 million doses of Pfizer and BioNTech's COVID-19 vaccine, which could receive emergency-use authorization from the US Food and Drug Administration as early as Friday evening.

"We could be seeing people getting vaccinated Monday, Tuesday of next week," Health and Human Services Secretary Alex Azar said Friday on "Good Morning America."

Azar's statement followed an endorsement on Thursday from an independent panel of experts - more than 20 leading scientists and doctors - who reviewed data from the trial of Pfizer and BioNTech's vaccine.

Moderna's vaccine is awaiting a similar panel review, which is scheduled for December 17. The FDA will then decide whether to grant it emergency-use authorization. In both cases, the agency is likely to follow the expert panel's advice.

Both Pfizer's and Moderna's vaccines use messenger RNA (mRNA): genetic material that tells cells how to make proteins. They work by injecting a small piece of mRNA from the coronavirus into the body that codes for the virus' spike protein. This protein helps the coronavirus attach to and invade cells, and it's what antibodies target and neutralize. The mRNA vaccine spurs the body to produce the spike protein internally, which triggers an immune response.

If its vaccine gets authorized, Moderna plans to deliver 20 million doses by the end of December, then distribute the rest of the first 100 million-dose order throughout January, February, and March. The additional doses purchased on Friday will be delivered in the second quarter of 2021, according to the press release.

"Securing another 100 million doses from Moderna by June 2021 further expands our supply of doses across the Operation Warp Speed portfolio of vaccines," Azar said, according to the release. "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."

The Moderna vaccine was codeveloped by the National Institutes of Health. In late-stage trials, it was found to be 94.5% effective at preventing symptomatic COVID-19.

Through Operation Warp Speed, the US has the option to order up to 300 million more vaccine doses from Moderna and 500 million from Pfizer.

However, The New York Times reported that the Trump administration did not take Pfizer's offer this summer to secure more doses for the US. So other countries got in line for batches of the vaccine and, as a result, Pfizer may not be able to deliver anything beyond the first 100 million doses to the US until June 2021.