The outgoing officer's first assistant can automatically fill the position for 120 days, at which point (or before) the President must put forth a nominee, or the President can appoint any other officer previously confirmed by the Senate.
Meanwhile, First Minister Carwyn Jones has paid tribute to outgoing Chief Medical Officer for Wales, Dr Tony Jewell, who is retiring after six years in the role.
In April MasterCard's board met in Singapore where it named Indian-born Ajay Banga, the company's chief operating officer and president, to succeed outgoing chief executive Robert Selander.