The Prime Minister’s announcement came at fundraising talks that raised £2.6bn for Global Alliance for Vaccines and Immunisation.
Britain’s pledge, by far the most generous from the countries at represented at the talks, will help vaccinate 800 million and save the lives of1.4 million children in the world’s poorest countries over the next five years.
The meeting had aimed to raise $3.7bn (£2.3bn), but months of lobbying headed by Microsoft founder Bill Gates managed to convince rich countries, businesses and charities to dig deeper.
"It was certainly thrilling at the outcome of this conference," said Gates afterwards.
"Over the last two months we were hoping people would come in and push themselves to be generous… That is exactly what happened,” he told the Guardian.
"For the first time we can say poor children will not be refused the vaccines that children in the richer world get because there is not enough money."
Mr Cameron faced criticism from the Tory’s right for ploughing so much into foreign aid, but said Britain’s contribution will mean is one child vaccinated every two seconds for five years. It is one child's life saved every two minutes. “That is what the money that the British taxpayer is putting in will give."
He said it was ‘unthinkable’ that in 2011, children are dying of preventable conditions such as pneumonia and diarrhoea.
"To those who say … we should put off seeing through those promises to another day because right now we can't afford to help, 'I say: we can't afford to wait.'"
Britain’s international development secretary, Andrew Mitchell, added that Immunising children "is absolutely brilliant value for money".
Drugs giants, selling some of the vaccines also promised to drop their prices on jabs destined for the world’s poorest countries. GlaxoSmithKline promised to cut a whopping 95 per cent off its rotavirus vaccine for sale to the developing world, while Merck also said it would cut the price on its vaccine against the same illness. Sanofi Pasteur and Johnson & Johnson also promised cuts. Gates stated:
"Vaccines are inexpensive, they are easy to deliver, and they are proven to protect children from disease,” said Mr Gates, who set up GAVI. He also stressed that all 193 countries who are members of the World Health Organisation need to put vaccines at the heart of their health systems.