U.S. Secretary of State Rex Tillerson delivers a keynote address at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson leaves after delivering remarks at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson leaves after delivering remarks at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson prepares to leave after delivering remarks at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson waves at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson waves at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson delivers a keynote address at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson delivers a keynote address at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

U.S. Secretary of State Rex Tillerson arrives at the 13th Annual Grand Challenges Meeting co-hosted by the Bill & Melinda Gates Foundation in Washington, U.S., October 4 2017. REUTERS/Yuri Gripas

Jerome Jarre, Juanpa Zurita, and Casey Neistat speak at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Raj Panjabi speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Stephen Fry speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Stephen Fry speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Queen Maxima of the Netherlands speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Stephen Fry and Isha Sesay speak at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Astro Teller speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Vittorio Colao speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Susan Desmond-Hellmann speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Trevor Noah speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Tom Ellman speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

George the Poet speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Nicholas Kristof speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Isha Sesay speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Bill Gates speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Leymah Gbowee speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Will.i.am speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Will.i.am speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Bill Gates speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Malala Yousafzai speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Will.i.am speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Will.i.am speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Bill Gates speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Canada's Prime Minister Justin Trudeau speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Melinda Gates and Canada's Prime Minister Justin Trudeau speak at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Canada's Prime Minister Justin Trudeau speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Melinda Gates and Canada's Prime Minister Justin Trudeau speak at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Canada's Prime Minister Justin Trudeau speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Canada's Prime Minister Justin Trudeau speaks at the Bill and Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Canadian Prime Minister Justin Trudeau and Melinda Gates attend the Bill & Melinda Gates Foundation Goalkeepers event in New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Canadian Prime Minister Justin Trudeau speaks at the Bill & Melinda Gates Foundation Goalkeepers event in New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Microsoft co-founder Bill Gates attends the Bill & Melinda Gates Foundation Goalkeepers event in New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama accompanied by Melinda and Bill Gates speaks at the Bill & Melinda Gates Foundation Goalkeepers event in New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama accompanied by Melinda and Bill Gates speaks at the Bill & Melinda Gates Foundation Goalkeepers event in New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Canadian Prime Minister Justin Trudeau speaks with Melinda Gates at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Melinda Gates applauds at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Melinda Gates is seen at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Canadian Prime Minister Justin Trudeau speaks at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Canadian Prime Minister Justin Trudeau speaks with Melinda Gates at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Canadian Prime Minister Justin Trudeau speaks at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Canadian Prime Minister Justin Trudeau speaks at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Canadian Prime Minister Justin Trudeau speaks at a Goalkeepers event hosted by the Bill and Melinda Gates Foundation in New York, U.S. September 20, 2017 REUTERS/Stephen Yang

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Bill Gates attends the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff

Former U.S. President Barack Obama speaks at the Bill & Melinda Gates Foundation Goalkeepers event in Manhattan, New York, U.S., September 20, 2017. REUTERS/Elizabeth Shafiroff