Vice President Joe Biden and Dr. Jill Biden thank nurses and doctors during a Christmas Day visit to Walter Reed National Military Medical Center, in Bethesda, Maryland, Dec. 25, 2014.

Vice President Joe Biden and Dr. Jill Biden thank nurses and doctors during a Christmas Day visit to Walter Reed National Military Medical Center, in Bethesda, Maryland, Dec. 25, 2014.

First Lady Michelle Obama, with family pets Sunny and Bo, visit with children of military families during a craft project in the State Dining Room of the White House, Dec. 3, 2014.

First Lady Michelle Obama, with family pets Sunny and Bo, visit with children of military families during a craft project in the State Dining Room of the White House,  Dec. 3, 2014.

President Barack Obama listens to Dr. Nancy Sullivan, Senior Investigator, Chief Biodefense Research Section, explain the investigational Ebola vaccine candidate currently being tested on humans during a lab tour at the Vaccine Research Center at the National Institutes of Health in Bethesda, Md., Dec. 2, 2014. Dr. Anthony Fauci, Director, National Institute of Allergy and Infectious Diseases and Dr. Francis Collins, Director, NIH watch in the background.

President Barack Obama listens to Dr. Nancy Sullivan, Senior Investigator, Chief Biodefense Research Section, explain the investigational Ebola vaccine candidate currently being tested on humans during a lab tour at the Vaccine Research Center at the National Institutes of Health in Bethesda, Md., Dec. 2, 2014. Dr. Anthony Fauci, Director, National Institute of Allergy and Infectious Diseases and Dr. Francis Collins, Director, NIH watch in the background.

President Barack Obama talks with White House Traveling Aide Bobby Schmuck as they walk to the Walter Reed National Military Medical Center landing zone for departure from Bethesda, Md., July 29, 2014. The President met earlier with 12 wounded warriors, ill or injured service members and their families.

President Barack Obama talks with White House Traveling Aide Bobby Schmuck as they walk to the Walter Reed National Military Medical Center landing zone for departure from Bethesda, Md., July 29, 2014. The President met earlier with 12 wounded warriors, ill or injured service members and their families.

First Lady Michelle Obama visits with families working on crafts at Maryland Fisher House IV at Walter Reed National Military Medical Center in Bethesda, Md., April 14, 2014.

First Lady Michelle Obama visits with families working on crafts at Maryland Fisher House IV at Walter Reed National Military Medical Center in Bethesda, Md., April 14, 2014.

President Barack Obama signs a Marine Corps flag after presenting a Purple Heart to HMC Richard Whitney, U.S. Navy, at Walter Reed National Military Medical Center in Bethesda, Md., March 17, 2014.

President Barack Obama signs a Marine Corps flag after presenting a Purple Heart to HMC Richard Whitney, U.S. Navy, at Walter Reed National Military Medical Center in Bethesda, Md., March 17, 2014.

Vice President Joe Biden and Dr. Jill Biden thank nurses during a Christmas Day visit to Walter Reed National Military Medical Center, in Bethesda, Maryland, Dec. 25, 2013.

Vice President Joe Biden and Dr. Jill Biden thank nurses during a Christmas Day visit to Walter Reed National Military Medical Center, in Bethesda, Maryland, Dec. 25, 2013.

President Barack Obama walks from Marine One to the waiting motorcade after arriving at the Walter Reed National Military Medical Center landing zone in Bethesda, Md., March 5, 2013. The President met with service members at Walter Reed National Military Medical Center, awarding two Purple Hearts and taking part in one promotion ceremony during his visit.

President Barack Obama walks from Marine One to the waiting motorcade after arriving at the Walter Reed National Military Medical Center landing zone in Bethesda, Md., March 5, 2013. The President met with service members at Walter Reed National Military Medical Center, awarding two Purple Hearts and taking part in one promotion ceremony during his visit.

Dec. 16, 2012 "The President works on his Newtown speech at an auditorium in suburban Washington. Two days earlier, I had photographed him when John Brennan first briefed him on the shootings. Throughout that day, he reacted as we all did, which people witnessed when he delivered his statement a few hours later. Before we headed to Newtown for the Sunday night vigil, he went to watch his daughter Sasha, 11, at her rehearsal for the Nutcracker; he would be unable to attend her performance because of the trip to Newtown. During breaks in the rehearsal, he worked on his speech. His expression in this photograph may be subtle to the viewer, but not to me. There is emotion and resolve etched on his face, and he knew the importance of this speech for the nation."

Dec. 16, 2012
"The President works on his Newtown speech at an auditorium in suburban Washington. Two days earlier, I had photographed him when John Brennan first briefed him on the shootings. Throughout that day, he reacted as we all did, which people witnessed when he delivered his statement a few hours later. Before we headed to Newtown for the Sunday night vigil, he went to watch his daughter Sasha, 11, at her rehearsal for the Nutcracker; he would be unable to attend her performance because of the trip to Newtown. During breaks in the rehearsal, he worked on his speech. His expression in this photograph may be subtle to the viewer, but not to me. There is emotion and resolve etched on his face, and he knew the importance of this speech for the nation."

Navy corpsmen help Vice President Joe Biden and Dr. Jill Biden into gowns before their meeting with patients and families during a Christmas visit to Walter Reed National Military Medical Center

Navy corpsmen help Vice President Joe Biden and Dr. Jill Biden into gowns before their meeting with patients and families during a Christmas visit to Walter Reed National Military Medical Center in Bethesda, Md., Dec. 25, 2011.