President Joe Biden was seen boarding Air Force One in Delaware on Tuesday, marking the first time he has been seen in public since being diagnosed with COVID-19 on July 17. 

The president held a mask in one hand as he gave onlookers a thumbs-up and a salute before disappearing into the plane.

"His walking on the stairs looks fine, better than usual, but it's strange that he didn't say hello to anyone," Dr. Marc Siegel, clinical professor of medicine at NYU Langone Medical Center and a Fox News medical contributor, told Fox News Digital. "All his energy went into walking."

The president is headed to Washington, D.C., where he is scheduled to make a speech to the American public on Wednesday night.

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Biden’s personal physician, Kevin C. O'Connor, D.O., released the latest update on the president’s health on Monday, July 22, in which he stated that the president had completed his 10th dose of Paxlovid and that his symptoms have "almost resolved completely."

There has been a renewed emphasis on the president’s health — both physically and cognitively — in recent weeks, stemming from his concerning performance at the June 27 presidential debate and culminating in his July 21 announcement that he is ending his campaign for re-election.

The first presidential debate of 2024 left many Americans, and even some traditional Democratic allies, wondering about Biden’s mental fitness for office based on what appeared to be his unclear train of thought and lack of coherence.

"President Biden's performance in [the] debate reignited longstanding discussions about his cognitive abilities and his overall leadership capacity," Dr. Brett Osborn, a Florida neurosurgeon who specializes in cognitive function, told Fox News Digital after the debate.

"Biden struggled to maintain a coherent narrative, often losing his train of thought mid-sentence, and providing vague, rambling and undecipherable answers to highly critical questions," Osborn continued, expressing his professional opinion.

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The doctor, who has not treated the president, also called out Biden’s frequent pauses, struggle to find words and reliance on the notes he was writing as points of concern. 

"President Biden lacks the mental sharpness required for the highest office in the land … especially amid [today's] current national and international crises," Osborn said.

The neurosurgeon also mentioned that the 46th president has undergone multiple surgeries for brain aneurysms.

There is a famous saying in neurosurgery, Osborn noted: "When the air hits your brain, you’re never the same." 

The doctor said, "It is medically probable that when he took office in 2020, he was afflicted by mild cognitive impairment (MCI), a gateway syndrome to Alzheimer’s disease, and potentially Parkinsonism (not necessarily Parkinson’s disease itself)."

After Biden’s announcement on Sunday that he was dropping out of the 2024 presidential race, doctors shared their thoughts about what that action meant in terms of his ongoing cognitive health. 

Given signs of his "ailing cognitive status, President Biden's decision to step down from the race seems prudent," said Osborn.

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"By stepping down, President Biden can prioritize his health and potentially mitigate further cognitive decline, while also ensuring the leadership role is filled by someone fully capable of handling the demanding responsibilities of the position."

Siegel also told Fox News Digital that he believes Biden’s decision may be best for the president’s health.

"The stress of the race could have definitely made cognition worse — but it will progress anyway if there is a cognitive issue, and fitness is a big concern," the doctor told Fox News Digital on Sunday. 

Siegel has said on multiple occasions that he has not examined or treated the president and that he cannot make any definitive diagnoses.

The president tested positive for COVID-19 on July 17, sparking concerns about the risk the virus poses to older adults.

The White House released an announcement of Biden’s positive COVID test, noting that the president had been vaccinated and boosted and was "experiencing mild symptoms."

His upper respiratory symptoms included "rhinorhea (runny nose) and non-productive cough, with general malaise," according to a statement on the White House's website.

Doctors shared their concerns about the potential long-term effect COVID could have on Biden.

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"Post-COVID syndromes and long COVID have been shown to affect underlying neurological conditions," Siegel warned. 

"This can also come from recurrent COVID."

Osborn noted that the virus has been shown to have potential long-term effects on cognitive function, often referred to as "brain fog." 

"These effects can include difficulties with memory, attention and executive function," he said. 

"In someone with pre-existing cognitive issues like President Biden, contracting COVID-19 could exacerbate these symptoms – via neuroinflammation – and lead to a more rapid decline."

As of July 22, Biden had received his 10th dose of Paxlovid.

"His symptoms have almost resolved completely," O’Connor, the president’s doctor, wrote in a July 22 briefing. 

"His pulse, blood pressure, respiratory rate and temperature remain absolutely normal. His oxygen saturation continues to be excellent on room air. His lungs remain clear."

Along with meeting the official requirements for president, Siegel and Osborn agreed that whoever holds the office should also be physically and cognitively healthy.

"It is extremely important," Siegel told Fox News Digital. "We have a right to it as a country."

The role of president involves making complex and high-stakes decisions, often under significant pressure, Osborn noted. 

"Cognitive health is essential for clear thinking, effective communication and sound decision-making, all things President Biden lacks," he said, sharing his viewpoint. 

Physical health is also essential, Osborn said, as the role also requires long hours, frequent travel and crisis management.

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"A healthy president is better equipped to handle the demands of the office and to lead the nation effectively through various challenges," he added.

In light of Biden’s announcement to exit the race, Siegel said the decision "should be accompanied by a full health disclosure."

He said, "They should admit to what they know and reveal full neurological results."

The situation highlights the need for thorough health evaluations for presidential candidates – and sitting presidents — to ensure they can perform their required duties, according to Osborn. 

"Routine testing – physical and mental – should be made non-negotiable instead of being thwarted by the governing party," he said. 

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In response to outreach from Fox News Digital, the White House press office stated that "health was not a factor" in the president's decision to withdraw from the race. 

"He looks forward to finishing his term and delivering more historic results for the American people," said the White House in its statement.