BETHESDA, Md. >> President Joe Biden visited some of the nation’s leading scientists on the frontlines of the fight against COVID-19 today, as he announced the U.S. will have enough supply of the vaccine by the end of the summer to inoculate 300 million Americans.

Biden toured the Viral Pathogenesis Laboratory at the sprawling National Institutes of Health complex just outside Washington that created the COVID-19 vaccine now manufactured by Moderna and being rolled out in the U.S. and other countries. He was also set to address the agency’s team of researchers and scientists, who have investigated treatments for COVID-19 and other dangerous diseases.

The U.S. is on pace to exceed Biden’s goal of administering 100 million vaccine doses in his first 100 days in office, with more than 26 million shots delivered in his first three weeks. Meanwhile, Biden is moving to ease supply bottlenecks and ensure the nation has enough of the current two-dose vaccines to protect 300 million Americans.

Biden announced today that the U.S. had secured contractual commitments from Moderna and Pfizer to deliver the 600 million doses of vaccine by the end of July — more than a month earlier than initially anticipated.

The pace of injections could increase further if a third coronavirus vaccine from drugmaker Johnson & Johnson receives approval from the Food and Drug Administration.

Speaking with Dr. Anthony Fauci, the nation’s top infectious-disease specialist, Biden emphasized that his administration is doing everything possible to increase the vaccine supply and the country’s capacity to deliver injections into arms.

“It’s been a hell of a learning process,” Biden said.

Biden, wearing a mask, used his remarks to criticize President Donald Trump, saying he inherited “no plan to vaccinate most of the country.”

“It is no secret that the vaccination program was in much worse shape than my team and I anticipated,” he said.

The Viral Pathogenesis Laboratory is led by Dr. Barney Graham, whose team made critical discoveries years ago that laid the groundwork for rapid development of that and other COVID-19 vaccines. Before the pandemic erupted, one of Graham’s research fellows, Dr. Kizzmekia Corbett, had been using those earlier findings to develop a vaccine for MERS, a cousin of COVID-19.

On the tour, Biden was shown the lab bench where researchers sequenced the virus and developed the precursor of the Moderna vaccine.

Armed with their prior research, Corbett and Graham had a head start when Chinese scientists shared the genetic map of the new coronavirus in January 2020. They already knew how to make spike proteins, which coat the surface of the new coronavirus and its MERS relative, that were stable enough to be used as a key vaccine ingredient.

Within days, the NIH had sent instructions to Moderna to brew up doses, and Corbett and her colleagues were setting up the key lab and animal tests that would eventually prove they were on the right track.