Biden just purchased 200 million additional doses of coronavirus vaccines - and we now have enough shots to immunize most Americans

• Biden's administration Thursday bought 200 million COVID-19 vaccine doses from Pfizer and Moderna.
• That marks a 50% increase in the US's available supply, bringing the total to 600 million doses.
• Biden said Trump "didn't order enough vaccines" while in office.

Days after taking office, President Joe Biden said his administration planned to buy another 200 million COVID-19 vaccine doses - 100 million from Pfizer and 100 million from Moderna. That purchase went through on Thursday, the president said, marking a 50% increase in the nation's total vaccine supply.

The US has now ordered 600 million total doses from the companies - enough to vaccinate 300 million Americans. That gives the country enough doses for all of its adult population: about 255 million people.

The new supply won't be delivered until the end of July, meaning it will help prevent vaccine shortages down the road but won't accelerate the pace of vaccinations.

While speaking at the National Institutes of Health in Bethesda, Maryland, on Thursday, Biden said he had inherited a vaccination program that was "in much worse shape" than his team had anticipated.

"My predecessor, to be very blunt about it, did not do his job in getting ready for the massive challenge of vaccinating hundreds of millions of Americans," Biden said. "He didn't order enough vaccines. He didn't mobilize enough people to administer the shots."

In his first three weeks in office, the Biden administration has increased the weekly supply of vaccines available to states by nearly 30%, the president said. About 34.7 million people in the US have received at least one dose of the two-dose vaccines.

"Millions more Americans will get vaccinated in February than the previous administration was on track to do," Biden said.

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, recently told "Today" that he expected the US to reach "open season" - the time when all adults have access to coronavirus shots - by April.

As new variants spread, public-health experts push for speedy vaccinations

Pfizer's and Moderna's vaccines are more than 90% effective at preventing symptomatic COVID-19 - the highest efficacy rates of any coronavirus shot thus far.

Both shots rely on messenger RNA (mRNA) to send a coded message to the body that triggers an immune response. Moderna's shot has been authorized for people 18 and older, while Pfizer's shot has been authorized for people ages 16 and up.

"The new mRNA technology - that's absolutely the Rolls Royce, the top of the line vaccines that you've seen come out," Dr. Johan Bester, the director of bioethics at the University of Nevada, Las Vegas, School of Medicine, told Insider.

But the companies are expected to deliver only 220 million doses in total to the US by the end of March. That's enough to vaccinate 110 million people - just 42% of those who are currently eligible to receive their shots.

Public-health experts are optimistic that vaccinations can speed up if the Food and Drug Administration authorizes Johnson & Johnson's coronavirus shot. The company last week asked US regulators to give its vaccine an emergency OK. An FDA advisory committee will meet to discuss the request on February 26, and the shot could be green-lighted a few days later.

Experts are adamant that people get vaccinated as quickly as possible, given the spread of new, more infectious variants like B.1.351, a coronavirus strain first identified in South Africa. Preliminary research found that B.1.351 could partially evade the protection offered by current vaccines.

"If presented with any one of the available vaccines, and you could get the other one in one month, I would go for the one in front of you right now," Dr. Becky Smith, an infectious-disease specialist at Duke Health, told Insider. "You just don't want to get COVID and they're all super effective."