UPDATE 1-Gates Appeals for "decade of Vaccines"

* In keynote speech to WHO, Gates lays down challenge

* Urges drug companies to make available affordable vaccines

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GENEVA, May 17 (Reuters) - Bill Gates called on Tuesday for strengthened immunisation programmes against infectious diseases to save 4 million lives by 2015 and 10 million lives by 2020 during a "decade of vaccines".

The Microsoft founder and philanthropist said five or six new vaccines could be available by the end of the decade and urged pharmaceutical manufacturers to make them affordable for poor countries.

"If donors are generous, we will prevent 4 million deaths by 2015. By 2020, we can prevent 10 million deaths," Gates, co-chair of the Bill and Melinda Gates Foundation, said.

He was addressing the annual assembly of the World Health Organisation (WHO) in Geneva, attended by health ministers from the U.N. agency's 193 member states.

All countries should aim to have 90 percent immunisation coverage against diseases including polio, meningitis and pneumonia, Gates said.

"We can meet these goals with your leadership. And that will be critical to really making this the decade of vaccines," he said. "It might be the most difficult thing we've ever done, but it will also be the most important." (Reporting by Stephanie Nebehay)