• President Joe Biden is poised to meet his goal of delivering 100 million Covid-19 vaccine shots in his first 100 days in office as early as Thursday, a senior administration official told NBC News.
• The president reached the goal ahead of schedule, the official said.
• Health experts say the president's goal of 100 million shots in 100 days was an attainable benchmark.

President Joe Biden is poised to meet his goal of delivering 100 million Covid-19 vaccine shots in his first 100 days in office as early as Thursday, a senior administration official told NBC News.

The president reached the goal ahead of schedule, the official said. Biden was sworn in as the 46th president of the United States on Jan. 20, about 57 days ago. Biden said last week that he expected to hit the goal on day 60.

Biden is scheduled to make an announcement on the "state of vaccinations" later Thursday where he may discuss the milestone.

Health experts say the president's goal of 100 million shots in 100 days was an attainable benchmark. After a slower-than-expected rollout under former President Donald Trump, the pace of vaccinations in the U.S. has rapidly increased, averaging about 2 million to 3 million shots per day.

Since taking office, the Biden administration has worked to ramp up the supply of vaccine doses in the U.S. after states complained that demand for the shots was outpacing supply.

Last week, the administration said it would buy 100 million additional doses of Johnson & Johnson's Covid-19 vaccine. The deal would double the nation's supply of the J&J vaccine, as the company already has a deal with the government to provide 100 million doses by the end of June. Merck is helping to make J&J's Covid vaccine.

The administration also has deals with drugmakers Pfizer and Moderna for 600 million doses combined, enough to inoculate 300 million Americans, since those two vaccines require two shots given three to four weeks apart.

Biden is directing states to make all adults, ages 18 and up, eligible for the vaccines by May 1, he announced a week ago. The administration in May will launch a website to help people find nearby vaccination sites, and the Centers for Disease Control and Prevention will issue new guidance about health and safety for those who have been vaccinated.

Though the pace of vaccinations is increasing, the administration still faces another problem: Vaccine hesitancy.

Even though clinical trial data shows the vaccines are safe and highly effective, just under half of adults in the U.S. surveyed in December said they were very likely to get vaccinated, according to a study from the CDC.

Public officials are also running into an unforeseen issue with distributing J&J's shots. Though J&J's vaccine is a highly effective shot, especially against severe illness and death, its efficacy rate is lower than Pfizer's and Moderna's and is perceived by some Americans as inferior as a result.

The administration also faces the threat of new, emerging variants. The CDC has said the B.1.1.7 variant first identified the U.K. is expected to become the dominant strain in the U.S. by the end of this month or in early April. A study published in the British Medical Journal found the highly contagious strain is associated with a 64% higher risk of dying from Covid-19 than earlier strains.

Top health officials including White House Chief Medical Advisor Dr. Anthony Fauci are urging Americans to get vaccinated as quickly as possible, saying the virus can't mutate if it can't infect hosts and replicate.

Correction: The headline on this story was updated to reflect that President Joe Biden's goal of having 100 million Covid vaccines administered during his first 100 days in office will be met as early as Thursday.