Healthcare workers operate in an ICU in the "COVID Area" of the Beverly Hospital in Montebello City, California, the United States, Jan. 22, 2021. (Xinhua)

"It is really a floor and not a ceiling," Fauci told CBS' "Face The Nation." "It is going to be a challenge. I think it was a reasonable goal that was set. We always want to do better than the goal that you've set."

However, a new model by scientists at Columbia University shows that "vaccines alone are not enough," said a report by New York Times on Sunday.

"The coronavirus pandemic in the United States has raged almost uncontrollably for so long that even if millions of people are vaccinated, millions more will still be infected and become ill unless people continue to wear masks and maintain social distancing measures until midsummer or later," said the report titled "Why Vaccines Alone Will Not End the Pandemic."

Moreover, the spread of a more infectious variant of the virus in the United States could make matters worse.

The new variant, first identified in Britain, has been detected in at least 22 U.S. states, according to data posted Friday by the Centers for Disease Control and Prevention.

A model developed by the Institute for Health Metrics and Evaluation at the University of Washington shows that the United States is projected to see more than 566,000 COVID-19-related deaths by May 1.

Taking into account multiple scenarios based on masking mandates, vaccine distribution and other behavioral changes, the model predicts that deaths in the country will not start leveling off until early March.