The U.S. government has purchased 200 million additional doses of Moderna's COVID-19 vaccine.

With the 300 million doses already purchased from the biotechnology company, that brings America's total order up to 500 million.

Under the terms of agreement, Moderna will deliver 110 million doses by the end of 2021 and 90 million in the first quarter of 2022. 

What's more, the Biden administration will also have the option to purchase any future coronavirus immunizations that Moderna is working on.  

It comes as the rate of vaccinations in the U.S. has slowed significantly to an average of 1.1 million shots per day, down from an average of 3.3 million in early April - making it highly unlikely the U.S. will meet President Joe Biden's goal of vaccinating 70 percent of all U.S. adults by July 4.  

It is not clear what the future doses will be used for but there are a few theories.

One is that the shots sitting in the U.S. stockpile may have expiration dates that are approaching and the new doses will replace those. 

Another is that, because scientists believe COVID-19 is going to become an endemic disease - meaning it is always circulating at low rates - all future generations will have to be vaccinated against it, and the doses are in preparation for when the vaccine is authorized in kids.

A third option could be that the doses are used as part of Biden's plan to deliver at millions of shots for about 100 countries.