Biden’s administration will purchase 100 million doses each of the vaccines made by Pfizer Inc and BioNTech, and Moderna Inc, increasing the overall total doses to 600 million, with delivery expected by summer. The previous purchase target was 400 million doses.

Each vaccine requires two doses per person to be fully effective, suggesting the new purchases would build up enough of a stockpile to inoculate most of the country’s 331 million people. The vaccines are not approved for use by most children.