The great achievements of vaccines are not consigned to the past.

It is easy to see the heroic age of vaccines as one that ended decades ago. The Salk polio vaccine, after all, which swiftly and visibly transformed the disease into a distant memory in the developed world, was introduced in 1955. And the smallpox eradication campaign led by the World Health Organization had, by the late 1970s, reduced the virus from a killer of millions of people a year to a prisoner of biosafety labs. These were monumental feats, but the best could be still to come.

This week Nature explores the undiminished promise of vaccines, and the factors that threaten it — complacency, funding shortages and the unease that vaccines provoke in so many people.

Online collection Worldwide, up to one-third of all deaths of children under five result from diarrhoea and pneumonia. In the past ten years or so, vaccines against the microorganisms that cause many of these cases have become a standard part of the childhood regimen in the developed world. If they could be made available worldwide, the lives of hundreds of thousands of children could be saved each year.

Research efforts are adding to the promise. Together, AIDS, malaria and tuberculosis kill more people each year than smallpox did when the global campaign to eradicate it began in 1967. The search for vaccines for all three diseases has been long and frustrating, but a Perspective on page 463 describes how new technologies are reviving it.

There is no room for complacency. The global campaign to eradicate polio made stunning progress from 1988 to the end of the twentieth century, reducing worldwide incidence by 99%. But the disease continues to smoulder in Pakistan, India, Afghanistan and Nigeria, where vaccinators have struggled with turmoil and corruption, high transmission rates and suspicion about the vaccine itself (see pages 427and 446). Similarly, a long vaccination campaign against measles has reduced the global death toll from more than 2.5 million a year in 1980 to fewer than 200,000 today. But vaccination rates are still below 80% in much of Africa and India, and funds pledged to the global measles initiative have fallen. Some people think that the disease is poised to surge again in the developing world (see page 434). Europe has already seen outbreaks, in part because vaccination rates dipped after the combined measles, mumps and rubella (MMR) vaccine was falsely linked to autism.

Vaccines can become victims of their own success. In the developed world, for example, vaccination has already reduced measles to a rarity, which makes an 'informed' choice to shun the vaccine seem risk free. Even doctors and nurses can fall prey to this reasoning. They have a disproportionate influence over whether parents vaccinate their children, and when they lose sight of the overwhelming ratio of benefit to risk for most vaccines, they can amplify public fears (see page 443). Back in the 1950s, '60s, and '70s, when vaccines offered protection against clear and present menaces, it was easier to accept their small risk of harm.

Designing a cheap, effective vaccine against the more complex major killers of today is a harder task, and people everywhere are quicker to question the official line, on vaccines as on everything else. But the promise for vaccines to transform global health is as bright as ever, and funders and public-health experts must continue their heroic support for research, global vaccination efforts and communication strategies to win over the doubters.

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