"People were worried about efficacy," he said, adding there is now real-world data showing how effective the vaccines are.

And finally, Swaminathan said, some Americans were concerned that the vaccines didn't have approval from the Food and Drug Administration and had only received emergency use authorization.

But Pfizer/BioNTech announced Friday they have initiated an application to the agency for full FDA approval of their vaccine for people 16 and older -- making it the first Covid-19 vaccine in the US to be assessed for full approval from the FDA.

The FDA "will move as expeditiously as possible," without compromising its safety standards, to assess Pfizer's Covid-19 vaccine for approval, White House Covid-19 response coordinator Jeff Zients said Friday.

More than 151 million Americans -- roughly 45.6% of the US population -- have received at least one Covid-19 vaccine dose, according to data from the Centers for Disease Control and Prevention.

J&J pause 'cast a shadow' on vaccinations

One factor that may have contributed to the slowing vaccinations was the recommended pause on the Johnson & Johnson Covid-19 vaccine, one expert said Saturday.

Over seven days, the J&J vaccine accounted for just about 3.5% of total doses administered, according to data published Friday by the CDC.

Boosters will likely be a reality, expert says

Meanwhile, the CDC is preparing for seasonal Covid-19 vaccine boosters in case they are needed, its chief said Friday, adding researchers are also looking into whether the US will need a booster that specifically works against certain coronavirus variants.

"We want to be prepared for them should we need them."

Potential Covid-19 boosters could be offered in a similar way to the annual flu vaccine, Walensky said.