Around 200 million doses will go out in 2021 and 300 million will be distributed in the first half of 2022.
It comes as US President Joe Biden said before leaving for the G7 summit in Cornwall that he would be announcing a vaccine strategy for the world.
The new order would almost double the government's original agreement to buy up to 600 million BioNTech/Pfizer doses.
The White House has been under mounting pressure to boost donations of COVID-19 shots to other countries.
The White House declined to comment, but a person familiar with the deal said the White House's COVID-19 response team had been negotiating its terms over the last four weeks. Moderna did not immediately respond to requests for comment.
"COVID-19 vaccines are a critical tool in overcoming this pandemic", said CDC Director Rochelle P. Walensky.
The United States has given at least one shot to around 64 per cent of its adult population and has begun vaccinating adolescents, while other countries like Brazil and India are struggling to get desperately needed doses. These groups are more likely to be exposed to the virus that causes COVID-19 due to their occupations.
Pfizer's vaccine has been authorized for use in children as young as 12 in Europe, the United States and Canada.
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