President Joe Biden says it will be difficult for the United States to reach herd immunity, at least 75 per cent of the population inoculated against coronavirus, before the end of August.
"The idea this can be done and we can get to herd immunity much before the end of ... this summer is very difficult," Biden told CBS news.
As of Sunday morning the US had administered 41,210,937 doses of COVID-19 vaccine and distributed 59,307,800 doses, the Centers for Disease Control and Prevention said.
Meanwhile, the nation's top infectious diseases expert, Anthony Fauci, warned second doses of vaccine should not be delayed so more people can get first shots.
"We must go with the scientific data that we've accumulated and it's really very solid," Fauci told NBC's Meet the Press.
"You can get as many people in their first dose at the same time as adhering, within reason, to the timetable of the second dose."
He was addressing comments from epidemiologist Dr Michael Osterholm discussing whether to stop stockpiling second doses so more Americans can get their first shots.
Vaccines developed by Pfizer and Moderna require two doses to be fully effective.
"One of the problems ... is that if you want to really study it to see that, the amount of time that it will take, the amount of people you would have to put into the study - by that time, we will already be in the arena of having enough," Fauci said.
He noted demand far exceeds supply following the rocky rollout of the vaccines in December.
"That will get better as we get through February and into March but that is the limiting factor," Fauci said.
An alarming new strain of the virus is rapidly spreading throughout the US, according to a new study.
The variant first found in Britain could become the predominant strain, potentially causing a new surge and increasing the risk of death, the New York Times noted.
"There could indeed be a very serious situation developing in a matter of months or weeks," epidemiologist Nicholas Davies of the London School of Hygiene and Tropical Medicine told the Times.
"These may be early signals warranting urgent investigation by public health authorities."
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