President Biden on Thursday slammed his predecessor for "not doing his job in getting ready for the massive challenge of vaccinating hundreds of millions of Americans."

Driving the news: Biden's remarks at the National Institutes of Health came not long after his administration signed final contracts with Pfizer and Moderna to purchase an additional 200 million doses of the coronavirus vaccines.

What he's saying: "While scientists did their job in discovering vaccines in record time, my predecessor — I’ll 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.

• Former President Trump "didn’t order enough vaccines. He didn’t mobilize enough people to administer the shots. He didn’t set up federal vaccine centers where eligible people could go and get their shots."
• "When I became president three weeks ago, America had no plan to vaccinate most of the country. It was a big mess."
• "It’s going to take time to fix, to be blunt with you. I promised, when I did my inaugural address, that I’d always be straight with you — give it to you straight from the shoulder: I will not walk away when we make a mistake; I’ll acknowledge it and tell you the truth."

The big picture: More than 34.7 million people in the U.S. have received at least one dose of the COVID-19 vaccine, according to the Centers for Disease Control and Prevention.

• Biden said that the U.S. is now on track to have enough supply of the vaccine to inoculate 300 million Americans by the end of July.
• Of note: It's unlikely Thursday's purchase will make the vaccine widely available sooner than originally planned, but it may prevent shortages later this year, per the Washington Post.

Go deeper: Coronavirus infections are plummeting