Dr Anthony Fauci has said he is cautiously optimistic there will be a Covid-19 vaccine by the end of the year or early 2021, but warned that the next few weeks will be critical to tamping down coronavirus hot spots around the country.

Fauci and other top health officials also said they have not been asked to slow down testing for coronavirus, an issue that became controversial after US President Donald Trump said last weekend that he had asked them to do just that because it was uncovering too many infections.

"We will be doing more testing,” Fauci, the government’s top infectious disease expert, told a House committee.

The US has tested more than 27 million people, with about 2.3 million – or 8.4% – testing positive.