Exactly a year ago today at a meeting hosted in London by David Cameron, we took a giant step to saving the lives of four million people in the world's poorest countries. Global political, business and philanthropic leaders came together to guarantee the means to vaccinate a quarter of a billion children against deadly diseases. It was a commitment based on simple concern for the most vulnerable, a belief in justice but also sound business sense.

For those of us who live in developed countries like Britain, it is easy to take for granted the incredible power of vaccines. It is a mistake you won't make if you see, as we both have, desperate mothers who have walked many miles to get treatment for children seriously ill with sickness such as diarrhoea and pneumonia. These are diseases which still kill over one million children every year yet we now have, in many cases, the vaccines to prevent.

Vaccines have transformed public health. Smallpox, which still killed two million people a year as late as 1967, has been eradicated from our world. Deaths from measles worldwide have been cut by 75 per cent over the last decade.

Vaccines are also highly cost-effective compared to the cost of medical treatment and the loss of potential and productivity through death and ill-health. For scarcely more than the price of a large cup of coffee, for example, a child can be vaccinated against five major childhood killers including diphtheria and tetanus.

These children will grow up to strengthen their economies and societies. It is why Bill Gates - a man who knows a great deal about sound investments - has spent billions of his own dollars on vaccinations which he sees as simply the best value-for-money buy in development.

It was to stop this individual misery and deliver these benefits that the Global Alliance for Vaccines and Immunisation was created. GAVI is an unprecedented public-private partnership between international agencies like the World Health Organization, UNICEF and the World Bank, governments in the developed and developing world, vaccine producers and philanthropic foundations such as the Bill & Melinda Gates Foundation.

The results have been dramatic. Since 2000, 325million more children have been vaccinated against a wide variety of diseases, saving more than five million precious lives. Britain was among GAVI's founders and remains an unwavering government backer. Britain's contribution alone means that every two seconds a child in the poorest countries will be immunised and a life saved every two minutes until 2015. When you are determined, as we are, to ensure maximum long-term benefit for every pound of aid, it makes absolute sense to put immunisation at the top of your agenda.

But while one in five children still miss out on routine vaccinations, we knew we could not rest. It was why there was such alarm last year that GAVI had a $3.7billion shortfall in its funds - and relief that this money and more was raised at the London conference.

A year on, what has been achieved? The answer is millions more children protected, new vaccines introduced against the biggest killers of children and costs cut to deliver maximum value for money.

Save the Children has estimated that the $1.1billion already paid in contributions since the conference will save an additional one million lives. This is well on the way to achieving GAVI's target by 2015 of preventing four million more early deaths.

Over the last 12 months, too, an increasing number of manufacturers have through GAVI reduced the price of vaccines for the poorest countries. Earlier this year, major suppliers agreed to cut by up to two-thirds the price of the rotavirus vaccine which protects children against severe diarrhoea.

This is all helping end the delay, which once was as long as 15 years, between vaccines being used in the wealthiest and poorest countries. Pneumococcal vaccines, which prevent the main cause of pneumonia, have already been rolled out in 15 African countries less than two years after their first use in Europe.

There is, of course, a huge amount more to do. This year, 1.7million children will still die from diseases we have the vaccines to prevent. With new vaccines being developed all the time, there are also fresh opportunities to save lives.

Health systems which enable developing countries to deliver effective vaccination and other public health programmes must be built up. More can still be done by using GAVI's funding to create economies of scale to lower the price of vaccines. But we are moving strongly in the right direction.

We will both be in Washington this week to help step up international momentum on improving children's health. The world must not waste the chance it has to eliminate preventable child deaths within a generation. Vaccines are not, of course, the only solution but they are, we believe, the cornerstone to achieving this ambition.

We are both fathers. Our children are fortunate to live in countries where they automatically get the protection that vaccines provide. All children, wherever they live, deserve to share in this miracle. British taxpayers through GAVI are helping make it happen.