Scott Scott Gottlieb, a former director of the US Food and Drug Administration (FDA), said on Sunday that the US may never achieve mass immunity against novel coronavirus, although the sharp drop in the number of new infections in the US this month is a sign of optimism and he expects the trend to continue.

"I really don't think we're going to achieve full mass immunity," Gottlieb said. "it's not going to disappear like measles or smallpox any time soon. Novel coronavirus will continue to spread at a low level. "

Although there are differences in estimates of the number of Americans who need to be vaccinated or have been infected with the virus, Dr Anthony Fauci, the chief US infectious disease expert, said in December that the figure might need to reach 90 per cent of the US population.

Other studies have found that if vaccination continues to accelerate, the United States could reach mass immunity as early as May.

Gottlieb said he was still "optimistic" about the future. As vaccination rates continue to rise, and 1/3 of the American public has been infected with novel coronavirus, it is possible to provide sufficient protection to continue to reduce the number of new infections, even if the United States does not achieve full mass immunity.