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 pledge from Saudi Arabia on Sunday to fight polio over the next five years, bringing it in line with the U.S. as one of the biggest national donors.

However, there is still a $1.2 billion dollar funding gap in the $4.8 billion budget for the Global Polio Eradication Initiative (GPEI) up to 2026, a spokesperson said. The new money from Saudi Arabia will go some way towards closing that.

Saudi Arabia has supported polio eradication for more than 20 years, but the significant increase in funding comes amid a “challenging” situation, said Abdullah Al Moallem, director of health at the King Salman Humanitarian Aid and Relief Centre, the kingdom’s aid arm.

Cases of polio, a viral disease that used to paralyse thousands of children every year, have declined by more than 99% since 1988 thanks to mass vaccination campaigns.

But the aim of getting cases down to zero, particularly in the two countries where the wild form of the virus remains endemic – Afghanistan and Pakistan – has been held up by insecurity in the regions where pockets of children remain unvaccinated.

“It’s not guaranteed that we will succeed,” Gates told Reuters in an online call last week. “I feel very strongly that we can succeed, but it’s been difficult.”

The support of powerful Muslim countries such as Saudi Arabia would help, he added, particularly in addressing some lingering suspicions about vaccination.

The foundation said it would open a regional office in Riyadh to support the polio and other regional programs.

It is allocating $4 million to humanitarian relief in Gaza, to be distributed through UNICEF, it said. The King Salman Humanitarian Aid and Relief Centre will also allocate $4 million, it said.

The first missed target for eradicating polio was in 2000, and the Bill and Melinda Gates Foundation is the largest donor trying to realise that goal.

“If we’re still here 10 years from now, people might be urging me to give up,” Gates said. “But I don’t think we will be. If things go well, we’ll be done in three years,” he said.

(Reporting by Jennifer Rigby, Editing by William Maclean)