LONDON - Success in the fight to wipe out polio is not guaranteed, according to tech billionaire turned philanthropist Bill Gates, whose foundation has poured billions into the effort.

Gates warned against complacency in tackling the deadly viral disease as he welcomed a $500 million (S$681 million) pledge from Saudi Arabia on Sunday (April 28) to fight polio over the next five years, bringing it in line with the U.S. as one of the biggest national donors.