(CNN) The Trump administration aimed to vaccinate 20 million Americans against Covid-19 by the end of 2020 but fell short of that goal even at the end of the term. Now, President Biden's promise to administer 100 million shots in his first 100 days in office will be put to the test.
More than 17.5 million Covid-19 vaccine doses have been administered since the first Pfizer/BioNTech shot was administered on December 14, according to the latest data from the CDC, for an average pace of about 462,000 shots per day.
While significantly behind early promises overall, the pace of vaccination has steadily improved over the past five weeks. The number of doses administered in the last seven days was more than 10 times that of the first seven days of vaccine administration in the United States. And with additional vaccines in the works, as well as commitments from the new Biden administration and current manufacturers, it could continue to improve.
In fact, Dr. Anthony Fauci said it's likely that vaccine administration will meet -- and perhaps even outpace -- President Biden's plan.
"I feel fairly confident that that's going to be not only that but maybe even better," the director of the National Institute of Allergy and Infectious Diseases said on Good Morning America on Thursday.
In any case, the US is still many months away from vaccinating enough people to end the pandemic.
The two vaccines currently authorized for emergency use in the US -- those made by Pfizer/BioNTech and Moderna -- both require that an individual receive two doses to be fully vaccinated. Another vaccine in development from Johnson & Johnson would require only a single dose.
If Pfizer/BioNTech and Moderna's vaccines remain the only ones on the market, about 500 million total doses must be administered before all adults in the US are fully vaccinated -- two for each of the 250 million or so adults who live in the US. Currently, none of the current coronavirus vaccines are authorized for people younger than 16.
In the past seven days, about 914,000 doses have been administered daily. If vaccination continues at this same rate, every adult in the US could be fully vaccinated by summer 2022, according to a CNN analysis.
If vaccination picks up to 1 million shots per day, in line with Biden's promise, that timeline could bump up to spring 2022.
To fully vaccinate all adults in the US by the end of the year, the pace would have to increase to about 1.3 million doses administered per day.
While vaccinating the full population is ideal, it's likely not necessary to start a return to normalcy.
Herd immunity -- when a sufficient proportion of a population is immune to an infectious disease and spread from person to person is unlikely -- would be expected to happen sooner.
Estimates for the share of the population that must be protected to reach this threshold vary as more is learned about the coronavirus. Most predictions land in the 60 to 70% range, including those cited by the World Health Organization.
But last week, Fauci indicated that range could be more like 70 to 85%.
"If we get that, we would develop an umbrella of immunity," he told CNBC. "That would be able to protect even the vulnerables who have not been vaccinated, or those in which the vaccine has not been effective."
Assuming three-quarters of US adults must be fully vaccinated to achieve herd immunity, the US could reach this threshold by February 2022 if vaccination continues at the same rate as the past seven days -- about 914,000 doses administered daily, according to a CNN analysis.
If vaccination picks up to 1 million shots per day, herd immunity in the US could be reached by the end of 2021.
At a Harvard Business Review event on Tuesday, Fauci said the effects of herd immunity may begin by fall.
"If we do that efficiently -- from April, May, June, July, August -- by the time we get to the beginning of the fall, we should have that degree of protection that I think can get us back to some form of normality," he said.
Despite recent reports that the US government does not have a stockpile of vaccine reserved for second doses as previously described, manufacturers have committed to the availability of supply.
"We have on hand all the second doses of the previous shipments to the US. We are working around the clock to produce millions more each day," Pfizer said in a statement to CNN.
Pfizer has committed to provide 200 million doses of Covid-19 vaccine by the end of July, and Moderna said it will deliver a total of 200 million doses by June, or the end of the second quarter of the yar.
Timelines for herd immunity and overall vaccination coverage could also accelerate if more single-dose vaccines are authorized for use in the US.
However, even if the United States reaches some level of herd immunity, the situation may be different in other parts of the world.
Last week, the World Health Organization's chief scientist, Soumya Swaminathan, warned that global herd immunity should not be expected this year.