The average number of vaccine doses being administered across the United States per day topped two million for the first time on Wednesday, according to data from the Centers for Disease Control and Prevention. A month ago, the average was about 1.3 million.

President Biden set a goal for the country shortly after taking office to administer more than 1.5 million doses a day, which the nation has now comfortably exceeded.

Mr Biden has also promised to administer 100 million vaccines by his 100th day in office, which is April 30. As of Thursday, 54 million people have received at least one dose of a Covid-19 vaccine. Johnson & Johnson’s one-shot vaccine was authorized for emergency use on Saturday, but those doses do not appear yet in the C.D.C. data.

The milestone was yet another sign of momentum in the nation’s effort to vaccinate every willing adult, even as state and city governments face several challenges, from current supply to logistics to hesitancy, of getting all of those doses into people’s arms.

Mass vaccination sites across the country are opening up or increasing their capacity, in part to respond to the new influx of doses from Johnson & Johnson. In New York, Gov. Andrew M. Cuomo announced on Thursday that three short-term mass vaccination sites will open in the state on Friday.

Three other state-run sites, including one at Yankee Stadium, will begin administering shots around the clock. In Georgia, Gov. Brian Kemp announced five new sites will open on March 17.

The Federal Emergency Management Agency has recently helped open seven mega-sites in California, New York, and Texas, that are staffed with active-duty troops. In Chicago, a vaccination site at the United Center will open next week, with a capacity of 6,000 shots a day. Many more such sites are planned.

There have been some hiccups in the massive logistical challenge of distributing millions of doses across the country, with special requirements for storage and handling.

In Texas, more than 2,000 doses went to waste over the past two weeks, according to an analysis by The Houston Chronicle. A majority of those losses were blamed on blackouts that swept the state in February, leaving millions of homes and businesses without power, some for multiple days.

And Mr. Biden has made equity a major focus of his pandemic response, saying he wants pharmacies, mobile vaccination units, and community clinics that help underserved communities to help increase the pace of vaccinations.

Experts say that Black and Latino Americans are being vaccinated at lower rates because they face obstacles like language barriers and inadequate access to digital technology, medical facilities, and transportation.

But mistrust in government officials and doctors also plays a role and is fed by misinformation that is spread on social media. In cities across the country, wealthy white residents are lining up to be vaccinated in low-income Latino and Black communities.

The president said on Tuesday that the country would have enough doses available for every American adult by the end of May, though he said it would take longer to inoculate everyone and he urged people to remain vigilant by wearing masks.

The administration also announced it had brokered a deal in which the drug giant Merck & Co. will help manufacture the new Johnson & Johnson vaccine. The unusual agreement between two rivals in the pharmaceutical industry was “historic,” Mr Biden said on Tuesday. “This is a type of collaboration between companies we saw in World War II.”

Mr Biden was also going to invoke the Defense Production Act, a Korean War-era law, to give Johnson & Johnson access to supplies for manufacturing and packaging vaccines.