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.

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.

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.

"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.

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.