Dr Hahn insisted there was a safe way to make a vaccine available before the end of Phase Three trials — potentially by issuing an emergency authorisation for use by certain groups rather than a blanket approval.

“Our emergency use authorisation is not the same as a full approval,” he said. “The legal, medical and scientific standard for that is that the benefit outweighs the risk in a public health emergency.”

But he insisted he would not rush a vaccine to please the president, amid reports that Mr Trump wants a vaccine to be available before November’s election to help improve his chance of victory.

“We have a convergence of the Covid-19 pandemic with the political season, and we’re just going to have to get through that and stick to our core principles,” he said. “This is going to be a science, medicine, data decision. This is not going to be a political decision.”