Biden delivered remarks announcing that the U.S. is "halfway" to his goal of administering 100 million vaccines in his first 100 days after he, Harris and Dr. Anthony Fauci, watched as four people received a COVID-19 vaccine.

"50 million shots in just 37 days since I’ve become president. That's weeks ahead of schedule, even with the setbacks we faced during the recent winter storms which devastated millions of Midwestern cities, towns, and also the same in the South," Biden said. "We're moving in the right direction, though, despite the mess we inherited from the previous administration."

Biden also worked to counter vaccine hesitancy, announcing that the administration is going to launch a "massive campaign" to educate Americans about the safety and efficacy of the vaccines.

"And there are folks who are hesitant to take the shot in the first place. We all know there is a history in this country of subjecting certain communities to terrible medical and scientific abuse," Biden said. "But if there is one message that needs to cut through, it's this: The vaccines are safe and effective"

Biden also pushed his COVID-19 relief bill, the American Rescue Act, and said he hopes Congress will pass it.

Despite the milestones that he touted, the president also offered a warning not to relax health measures.

"So I want to make something really very clear: this is not a time to relax," Biden said. "We must keep washing our hands, stay socially distanced, and for god's sake, wear a mask."

ABC News' Senior White House Correspondent Mary Bruce pressed White House press secretary Jen Psaki in a White House briefing Thursday about the president's plans to hold accountable those responsible for the death of journalist Jamal Khashoggi.

Psaki wouldn’t offer specifics but didn’t rule out sanctions, saying “a range of actions” are on the table.

Psaki stressed that the next step is for Biden to have a call with the King of Saudi Arabia, Abdul Aziz, and for the intelligence community to release its full report of the events.

"And, of course, our administration is focused on recalibrating the relationship, as we've talked about in here previously, and certainly there are areas where we will express concerns and leave open the option of accountability," Psaki said.

Bruce also asked why the president would not speak directly with Crown Prince Mohammed bin Salman, given not only his role in the future leadership of the country but also his reported involvement in the Khashoggi case.

Psaki pointed to bin Salman’s conversations with his counterpart, Secretary of Defense Llyod Austin, but did not say if Austin planned to discuss the issue with him directly.

-ABC News' Molly Nagle

Harris visited a pharmacy in a Giant supermarket in a predominantly minority neighborhood of Washington, D.C., on Thursday in an effort to ease vaccine hesitancy on the same day she and Biden will tout reaching the 50 million-shot marker, which is halfway to their 100 million shots in 100 days goal. But on the same day of Harris' visit, there were problems with the district's website to register for vaccine appointments.

D.C. Mayor Muriel Bowser said the website issues were caused by a crush of demand after eligibility was expanded to include people with pre-existing conditions like severe obesity, asthma, liver disease and other conditions.

At the event, Harris spoke to Brenda Thompson, a D.C. resident who was about to receive her second dose, about her experience getting the Moderna vaccine, with Harris saying she needed to "take it a little slow" the day after her second dose.

Harris spoke with the store's pharmacist, Samir Balile, who told Harris he sees lines of people every day waiting for their shots, but he is concerned about hesitancy in the coming weeks.

Harris reported to him feeling some side effects after the second dose.

"The first dose, I was fine. The second dose, I thought I was fine, got up early in the morning, went to work, then midday I realized I might need to slow down a bit. Just that one day, and then it was fine, it was like nothing," Harris said.

-ABC News' Sarah Kolinovsky and John Parkinson

Biden and Harris will participate in an event Thursday afternoon commemorating 50 million COVID-19 vaccine shots administered during the Biden administration, which is halfway to Biden’s goal of distributing 100 million COVID-19 shots in his first 100 days in office.

When Biden set out the goal at the beginning of his term, the administration was already on pace to administer nearly 1 million vaccines a day, which some public health experts have said is not fast enough to control the pandemic.

Dr. Anthony Fauci, Biden’s chief medical adviser, has estimated that 325 million people will need to be immunized for the pandemic to end and to reach that threshold, Biden would need more vaccinations.

Biden has rejected the suggestion that his goal was too low, telling reporters in January that, "it's a good start."

Meanwhile, Harris visited a Giant supermarket in Southeast Washington, D.C. Thursday morning to promote the administration’s Federal Retail Pharmacy Program for COVID-19 vaccinations, which has delivered 1 million doses to over 6,500 pharmacies across the country.

Harris spoke to the store's pharmacist and then watched as a patient, Brenda Thompson, received her second dose of the Moderna vaccine.

Biden will also participate in the National Governors Association’s Winter Meeting Wednesday afternoon, and the White House will give a press briefing at 12:30 p.m.

After the U.S. conducted a military airstrike in eastern Syria along the border with Iraq on Thursday, White House press secretary Jen Psaki defended the president's authority to carry out the strike amid bipartisan pushback from members of Congress.

"As a matter of domestic law, the president took this action, pursuant to his Article II authority to defend U.S. personnel. The targets were chosen to correspond to the recent attacks on facilities, and to deter the risk of additional attacks over the coming weeks," Psaki said. "As a matter of international law, the United States acted pursuant to its right of self-defense, as reflected in Article 51 of the U.N. charter."

Psaki said the move was "necessary" and "proportionate" and reflected the president's unambiguous commitment to protecting American lives.

The Department of Defense briefed congressional leadership before the action Thursday night, according to a spokesperson on the National Security Council. The administration has been briefing lawmakers at the member and staff level Friday. There will be a full classified briefing early next week, and sooner if Congress wants, according to the spokesperson.

-ABC News' Molly Nagle