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.

Garland described the Jan. 6 insurrection at the Capitol as "the most heinous attack on the democratic processes I have ever seen and one I never expected to see in my lifetime." He added he will make sure to provide career prosecutors all the resources they need to carry out their investigations while also taking a broader look at the symptoms behind the country's domestic extremism problem.

Asked by Sen. Dianne Feinstein, D-Calif., whether the president "has the absolute right to do what he wants with the Justice Department," Garland said presidents are "constrained by the Constitution as are all government officials" and cited comments by Biden committing to not interfere with Justice Department matters. At the same time, Garland said that the Department of Justice is part of the executive branch and because of that, on policy matters they do "follow the lead of the president and the administration as long as it is consistent with the law." When asked who an attorney general represents when his interests conflict with the president's, Garland said the attorney general "represents the public interest, particularly and specifically as defined by the Constitution and the statutes of the United States."

Asked whether the president can order an attorney general to open or close an investigation, Garland said such a question was a hard one for constitutional law but that he did not expect it to be a question for himself given President Biden's statements assuring independence for the department.

Sen. Lindsey Graham, R-S.C., took a contentious tone with Garland in his line of questioning as he pressed him on multiple topics. At one point, when Graham asked Garland whether he thought former FBI Director Comey was a good FBI director, Garland declined to answer, which Graham said he found "stunning" because he thought Comey was terrible. Sen. Sheldon Whitehouse, D-R.I., pressed Garland on whether he would commit to investigating not only the rioters who attacked the Capitol on Jan. 6, but those "upstream" like the funders, organizers, ringleaders or others not actually at the Capitol.  Garland cited his past experience as a line prosecutor, noting "we begin with the people on the ground and we work our way up to those who are involved and further involved -- and we will pursue these leads wherever they take us."

Asked about whether he would end the Trump Justice Department's policy of generally stonewalling in the face of oversight requests from Congress, Garland committed to Whitehouse that the department would be "as responsive as possible" to any requests and "at the very least why if it can't answer a question or can't answer a letter." Garland also committed that he would work with Whitehouse on getting answers to the committee on requests that the Justice Department under Attorneys General Bill Barr and Jeff Sessions previously ignored.

In an exchange with Sen. John Cornyn, R-Texas, Garland committed that his personal politics will have no impact on prosecutions and investigations he oversees as attorney general. Asked what he would do if he was ordered to do something that he considered to be unlawful, Garland said he would first tell the president or whoever else was asking him that what they were ordering was unlawful and would resign if no alternative was accepted.

Asked by Sen. Amy Klobuchar, D-Minn., what he will do to improve morale in the department, Garland said he would on his first day make an oath to career prosecutors and agents "that my job is to protect them from partisan or other improper motives." Klobuchar then asked Garland whether he believes he'd need "additional authorities" to combat the country's domestic terrorism problem. Garland said while the department "is probably always looking for new tools ... the first thing we have to do is figure out whether the tools that we have are sufficient."

-ABC News' Alexander Mallin