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Jemima Khan: We are a wealthy country, so we can afford to help

The cost of a vaccine for one of the major child killers is less than most of us would pay for a daily cup of coffee

The cautious Prime Minister is being uncharacteristically brave. World leaders meet today in London for a Dfid-hosted pledging conference for Gavi (The Global Alliance for Vaccinations and Immunisation). Britain is taking the moral lead, not in rallying support for yet another military intervention but in lobbying for immunisation for every last child – the remaining fifth of the world's child population – against vaccine-preventable diseases. Cameron risks angering many of his backbenchers by announcing a major donation to Gavi's global child immunisation campaign, as part of the UK government's unique commitment to honour its pledge made at the 2005 G8 summit to give 0.7 per cent of national income to overseas aid.

Meanwhile, pharmaceutical companies have announced they will be selling their vaccines at significantly reduced prices (up to 95 per cent) to the developing world.

Cameron will be lambasted by the right of his party who say that charity starts at home, aid doesn't work and we can't afford it. It comes as no surprise that Liam Fox, the minister who presides over one of the most wasteful, worst-run, profligate and remedially managed departments in government, has emerged (after his "leaked" memo) as Cameron's big critic on this issue.

Many of these critics see no inconsistency in their continued support for military involvement in Afghanistan, a country that has never threatened Britain and which costs us £4.5bn a year. A conflict which has killed 307 of our servicemen and 30,000 innocent civilians. Somehow that is money well spent, whereas the £2.3bn that is needed from the world's wealthiest donor governments to vaccinate one quarter of a billion of the world's poorest children, saving four million extra lives, is a waste. Immunisation is one of the biggest success stories of the past three decades.

After clean drinking water, vaccines have saved more lives than any other public health intervention in modern history. Over 80 per cent of children globally are being immunised every year and 2.5 million lives saved annually.

Immunisation has eradicated smallpox and polio in all but four countries worldwide. Now vaccines exist for the two viruses that cause the biggest child killers, pneumonia and diarrhoea. Combined, these account for two thirds – one million a year – of all childhood deaths. Vaccines against HIV and malaria are in the testing and development stage and they could revolutionise global healthcare.

For those for whom the ethical argument is not sufficiently convincing, there's an economic incentive. Immunisation is highly cost-effective – it's far cheaper than treatment.

With the cooperation of the pharmaceutical companies, the cost of a vaccine for one of the major child killers is less than most of us would pay for a daily cup of coffee. The cynical self-interested point of view also holds that aid means trade. There is an incalculable impact on the economy when parents have to stay at home to care for a sick child.

The faster developing countries develop, the more stable they become and the more of our goods and services they will buy. Of course, it's not always easy. Aid has been misused by corrupt governments. That's why International Development Secretary Andrew Mitchell carried out a thorough review of aid effectiveness .

The most competent, accountable agencies, like Unicef, have been identified, meaning that the benefits of aid can be monitored and maximised. Success depends on transparency. There are those that will say that we can't afford it and anyway it's OK for me. But is it OK that a child who is born in one of the poorest countries is 20 times more likely to die before his or her fifth birthday than a child in the UK? Is it OK that one child dies unnecessarily every 20 seconds from a preventable disease?

Despite the economic crisis, the UK remains one of the wealthiest economies in the world.

With our reputation and credibility at an all-time low in many parts of the world at the moment, it's a good time to demonstrate that it's not just military intervention that interests us and that charity does not start and end at the white cliffs of Dover.

Jemima Khan is an ambassador for Unicef UK