Answering growing frustration over vaccine shortages, President Joe Biden has announced that the US is ramping up deliveries to hard-pressed states over the next three weeks and expects to provide enough doses to vaccinate 300 million Americans by the end of the summer or early fall, AP reports.

Biden, calling the push a “wartime effort,” said the administration was working to buy an additional 100m doses of each of the two approved coronavirus vaccines. He acknowledged that states in recent weeks have been left guessing how many vaccines they will have from one week to the next.

The administration said it plans to buy another 100m doses each from drugmakers Pfizer and Moderna to ensure it has enough vaccine for the long term. Even more vaccine could be available if federal scientists approve a single-dose shot from Johnson & Johnson, which is expected to seek emergency authorisation in the coming weeks.