WASHINGTON: The United States was among the biggest backers of the AstraZeneca Covid vaccine, but a series of blunders by the company eroded American authorities' confidence and have delayed rollout in the world's hardest-hit country.

The British-Swedish drugmaker was on the back foot Tuesday, vowing to release more data "within 48 hours" after the US National Institute of Allergy and Infectious Diseases (NIAID) raised concerns that results reported from its US trial were outdated.

The Washington Post meanwhile reported that independent experts tasked with overseeing the trial had earlier written a letter to AstraZeneca telling them they should report their efficacy as 69 to 75 percent, not 79 percent as the company had done when it omitted the most recent data.

"It's hard to understand what rationale could justify reporting stronger numbers when weaker and more up to date numbers were actually available," Holly Fernandez Lynch, an assistant professor of medical ethics at University of Pennsylvania told AFP, though she added that she was heartened by the government's quick response.