A passenger walks toward the airbridge as another walks up to use biometric boarding for an international flight United Airlines flight at IAH George Bush Intercontinental Airport amid the coronavirus disease (COVID-19) outbreak in Houston, Texas, U.S., July 21, 2020. REUTERS/Adrees Latif

A man wearing a protective mask and clothing as patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

A soldier is pictured as patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

Patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

Patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

Patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

Patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

Patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

Patients infected with coronavirus are stretchered into a military A400M aircraft during a transfer operation from Colmar airport to Toulouse, as the spread of the coronavirus disease (COVID-19) continues, Colmar, France, April 5, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A health worker wearing a protective gear takes a break during transfer operations of patients infected with coronavirus disease (COVID-19) from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann TPX IMAGES OF THE DAY

A health worker wearing a protective gear looks on during transfer operations of patients infected with coronavirus disease (COVID-19) from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A health worker wearing a protective gear takes a break during transfer operations of patients infected with coronavirus disease (COVID-19) from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A firefighter wearing a protective gear takes a break during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

Members of the medical staff are seen as patients, infected with coronavirus disease (COVID-19), are carried into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is seen inside a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A member of the medical staff from the firefighters looks on as patients, infected with coronavirus disease (COVID-19), are carried into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann TPX IMAGES OF THE DAY

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

Firefighters wearing protective suits get ready before the flight, during transfer operations of patients, infected with coronavirus disease (COVID-19), from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A soldier waits for a patient, infected with coronavirus disease (COVID-19), to be carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

A patient, infected with coronavirus disease (COVID-19), is carried on a stretcher into a Caiman helicopter from the French army during transfer operations from Strasbourg to Germany and Switzerland, France March 30, 2020. REUTERS/Christian Hartmann

Pope Francis descends the stairs of the papal plane after arriving in the United States at Joint Base Andrews outside Washington September 22, 2015. REUTERS/Tony Gentile

Pope Francis descends the air stairs from his aircraft to be welcomed by U.S. President Barack Obama (not pictured) upon his arrival at Joint Base Andrews outside Washington September 22, 2015. REUTERS/Kevin Lamarque

Pope Francis arrives for the first time in the United States at Joint Base Andrews, Maryland September 22, 2015. REUTERS/Kevin Lamarque

An Air Force personnel keeps watch as a crew member removes the passenger stairs from Air Force One after U.S. President Barack Obama's arrival at Joint Base Andrews in Washington September 14, 2015. Obama traveled to Des Moines, Iowa to hold a town hall to discuss college access and affordability. REUTERS/Carlos Barria

U.S. President Barack Obama salutes as he disembarks from Air Force One at Joint Base Andrews, Maryland, August 25, 2015. President Barack Obama, fresh from vacation and nine rounds of golf in MarthaÕs Vineyard, launched into a busy two weeks promoting renewable energy and his nuclear deal with Iran. REUTERS/Carlos Barria

U.S. President Barack Obama arrives aboard Air Force One at Joint Base Andrews, Maryland, August 25, 2015. President Barack Obama, fresh from vacation and nine rounds of golf in Martha's Vineyard, launched into a busy two weeks promoting renewable energy and his nuclear deal with Iran. REUTERS/Carlos Barria TPX IMAGES OF THE DAY

U.S. President Barack Obama waves as he boards Air Force One on the way back to Washington at the McCarran International Airport in Las Vegas, Nevada August 25, 2015. REUTERS/Carlos Barria

U.S. Secretary of State John Kerry walks up the stairs as he boards his aircraft from Andrews Air Force Base for a flight to Havana, Cuba, August 14, 2015. Kerry declared a new era in relations as he celebrated restored diplomatic ties in Havana on Friday, but he also urged political change in Cuba, telling Cubans they should be free to choose their own leaders. REUTERS/Pablo Martinez Monsivais/Pool

U.S. President Barack Obama arrives aboard Air Force One at Jomo Kenyatta International Airport in Nairobi July 24, 2015. U.S. President Barack Obama flew into Kenya late on Friday for his first presidential visit to his father's homeland, aiming to boost trade and security ties in east Africa. REUTERS/Thomas Mukoya

U.S. President Barack Obama arrives to board Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst

U.S. President Barack Obama boards Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst

U.S. President Barack Obama waves as he boards Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst TPX IMAGES OF THE DAY

U.S. President Barack Obama boards Air Force One for travel to Kenya and Ethiopia from Joint Base Andrews, Maryland July 23, 2015. Obama will land in Kenya on Friday with a mission to strengthen U.S. security and economic ties, but his personal connection to his father's birthplace will dominate a trip that Kenyans view as a native son returning home. REUTERS/Jonathan Ernst

Passengers board a Ukrainian airplane at Bassel al-Assad international airport in Latakia, May 24, 2015. Syria welcomed its first commercial flight from Europe in three years as an Autolux flight from the Ukrainian capital Kiev arrived in the government-held coastal city of Latakia on Sunday. Only 10 passengers arrived in Syria on the flight with return tickets costing around $600 per person. Picture taken May 24, 2015. REUTERS/Omar Sanadiki TPX IMAGES OF THE DAY

Yemenis, who were stranded in Egypt, pray as they disembark from their plane after returning home, at Sanaa international aiport May 20, 2015. Some 200 Yemenis who have been stuck in Egypt and unable to return home arrived in the capital Sanaa on Wednesday in the first such flight from Egypt, though many more are expected in the coming days and weeks, officials say. REUTERS/Khaled Abdullah

A staircase for U.S. Republican presidential nominee and former Massachusetts Governor Mitt Romney to be used to step off his campaign plane sits on the tarmac prior to his arrival at the site of a campaign rally in Dubuque, Iowa, November 3, 2012. REUTERS/Jim Young (UNITED STATES - Tags: POLITICS ELECTIONS USA PRESIDENTIAL ELECTION)

An employee dusts off the red carpet on the tarmac of Berlin's Tegel airport, September 22, 2011. Pope Benedict XVI will have a four-day visit to Germany beginning September 22. REUTERS/Kai Pfaffenbach (GERMANY - Tags: POLITICS RELIGION)

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U.S. President Barack Obama boards Air Force One before he departs Joint Base Andrews outside Washington for Michigan, August 11, 2011. REUTERS/Larry Downing (UNITED STATES - Tags: POLITICS TRANSPORT)

U.S. President Barack Obama prepares to board Air Force One before he departs Joint Base Andrews outside Washington for Michigan, August 11, 2011. REUTERS/Larry Downing (UNITED STATES - Tags: POLITICS ENVIRONMENT TRANSPORT IMAGES OF THE DAY)

U.S. singer Lady Gaga steps off her private jet at Sydney Airport July 9, 2011. Lady Gaga is in Sydney for a one-off show at Sydney's Town Hall on Wednesday. REUTERS/Dean Lewins/Pool (AUSTRALIA - Tags: ENTERTAINMENT IMAGES OF THE DAY)

Malia Obama, elder daughter of U.S. President Barack Obama, steps aboard Air Force One in Rio de Janeiro, March 21, 2011. The Obamas are travelling to Santiago, Chile, for the second stop on his Latin American visit. REUTERS/Jason Reed (BRAZIL - Tags: POLITICS)

U.S. President Barack Obama boards Air Force One at Andrews Air Force Base, near Washington, November 23, 2010. REUTERS/Jim Young (UNITED STATES - Tags: POLITICS)

Singapore's Prime Minister Lee Hsien Loong looks out from a window of an aircraft upon his arrival at Noi Bai airport in Hanoi October 27, 2010. Lee is in Hanoi to attend the 17th ASEAN summit running from October 28 to 30. REUTERS/Kham (VIETNAM - Tags: POLITICS)

Bo, the pet dog of U.S. President Barack Obama and his family, steps off Air Force One at Andrews Air Force Base near Washington, August 15, 2010. The first family and Bo returned to Washington after spending the weekend in Panama City Beach, Florida. REUTERS/Jason Reed (UNITED STATES - Tags: POLITICS ANIMALS IMAGES OF THE DAY)

Pope Benedict XVI waves from the platform at the top of the stairs before boarding his plane to Portugal at Fiumicino international airport in Rome May 11, 2010. REUTERS/Alessandro Bianchi (ITALY - Tags: RELIGION TRANSPORT)