In April, the UK saw its first measles-related death since 1992. Stephen Phillips charts a worrying fall in vaccine uptake, in particular among the well-heeled

For immunologists, it is the best and the worst of times. UK experts are consulting on whether to add another vaccine to the list of childhood jabs after a US government advisory panel recommended that all American females aged between 9 and 26 receive the first vaccine to protect against cancer. The new vaccine protects against strains of human papilloma virus (HPV), the cause of the most common sexually transmitted disease that is responsible for 70 per cent of cases of cervical cancer, the second most common cancer among women.

The number of vaccines has risen fast in the past few years. In the US, the new vaccine will bring to 16 the number of immunisations US children are expected to undergo. In 1995, there were only ten vaccine-preventable diseases; a decade earlier, seven. In the UK, children are routinely vaccinated against 12 diseases.

But many in the field fear that vaccinations have become victims of their own success.

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"It's a great irony," says Stephen Cochi, a senior immunisation adviser at the Centers for Disease Control and Prevention (CDC), the US Government's public health arm. "The implementation of vaccines was one of the 20th century's great public health triumphs. But because these diseases have been largely defeated, parents and even doctors don't see them very often, and there's a focus, almost an obsession, with real and imagined adverse effects."

Growing numbers of people around the world are refusing vaccines. The ethical implications of this trend are the subject of a major conference this week, held at Seattle's Children's Hospital. Experts will discuss how doctors should balance respecting parental wishes with the community's interests and whether doctors are ever justified in striking parents off their register.

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It is estimated that between 5 and 10 per cent of US parents have "objections" to vaccinations, while 2 to 3 per cent refuse to let their children have any vaccinations.

In the UK, attention has focused primarily on the measles, mumps and rubella vaccine. Last month, a dramatic rise in the number of British measles cases so far this year - including, in April, the first measles-related death since 1992 - prompted leading paediatricians and vaccination experts to write a sternly worded open letter urging the press, politicians and health officials to "draw a line under the question of any association between the MMR vaccine and autism".

The association was first made in a now discredited 1998 article in The Lancet, which most of its authors have since rejected and that has been debunked by subsequent studies. Despite the science, though, some parents still believe there is a link.

The MMR immunisation rate among two-year-olds in the UK was 83 per cent last June, up from a low of 78.9 per cent in June 2003, but still well below the 95 per cent coverage required for "herd immunity".

"We are now faced with a potentially serious situation," the letter reads.

"Years of low uptake mean large numbers of unprotected children are now entering school. Unless this is rectified urgently, there will be further outbreaks and more unnecessary deaths."

"It is time that due weight is given to the overwhelming body of scientific evidence in favour of the vaccine," the letter adds.

The General Medical Council is investigating Andrew Wakefield, the lead author of the 1998 paper and former Royal Free Hospital gastroenterologist, for alleged research improprieties and ethical breaches.

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In 2004, it emerged that Wakefield - who stands by his findings - had been retained by lawyers representing parents looking to find evidence that their children's autism was caused by the MMR vaccine when he conducted the study.

In the US, despite early concerns about the MMR vaccination, worries have focused on thimerosol, a mercury-based preservative used in routine childhood vaccines until 2001. In April, autism advocacy groups took out a full-page advertisement in the daily newspaper USA Today accusing the CDC of "creating an epidemic of autism" by sanctioning thimerosol's use in vaccines in the 1990s.

In 2001, the agency removed thimerosol from all vaccines, except some flu shots, strictly as a precautionary measure amid concerns that high levels of mercury in food can cause brain damage and affect the kidneys and immune system.

Numerous studies have found no link between thimerosol and autism, however. Cochi says most scientists view the rise in the number of autism cases - which has not abated after thimerosol's removal - as being due to better detection and diagnosis.

Experts blame vaccination resistance on a growing consumer mentality among patients (Cochi says that refusers tend to be middle class with a higher-than-average level of education), the viral power of the internet in propagating fringe theories and scaremongering media coverage.

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Paul Offit, vaccinology and paediatrics professor at the University of Pennsylvania, speaks of a process by which press reports of a controversial single study sow public concern, even if it is subsequently retracted. "It's hard to un-ring the bell," he observes.

Edgar Marcuse, professor of paediatrics at the University of Washington, points to the difficulty of explaining scientific thinking to the public. It is impossible for scientists to categorically rule out safety concerns. "All they can say is there's no evidence," he says.

Media notions of balance in faithfully presenting both sides may also lend weight to marginal hypotheses that command little academic kudos, he adds.

But even if public objections are misguided, in most cases they ought to be respected, says Douglas Diekema, associate paediatrics professor at Washington.

"In a liberal democracy, the law is based on the harm principle - that the state shouldn't interfere with decisions unless someone is put at risk.

Most of the time that's not going to be the case (with vaccine refusal) in a well-immunised community."

That doesn't mean parents shouldn't be informed that they are hitching a "free ride" on the group protection afforded by others who have been vaccinated or that their choice is not risk free, says Joel Frader, professor of pediatrics, medical humanities and bioethics at Northwestern University.

Diekema, for example, pointed out in the journal Pediatrics last year that, "although the risk of encephalopathy (brain disorder) related to the measles vaccine is 1 in 1 million, the risk of encephalopathy from measles illness is 1,000 times greater".

It is also legitimate, adds Frader, to flag up the possible risk to others who, because of cancer, HIV or an otherwise compromised immune system - or because they have had a previous allergic reaction - cannot be vaccinated; or those who, despite vaccination, remain susceptible to diseases - as many as 5 to 15 per cent of people.

"It's not unreasonable for the community to make it a little difficult, putting (parents) in a position where they have to defend a decision that may put others at risk," Diekema says.

But some doctors may be taking a harder line. In a 2002 poll, 39 per cent of US doctors said they would "dismiss" a family resisting all vaccinations, and 28 per cent said they would cut off families who refused individual vaccines. In the UK, there is anecdotal evidence that doctors are also striking off parents who refuse vaccinations.

"It is probably a more frequent problem than people have been talking about," suggests Frader, who believes that doctors should instead take more time to explain the science to parents.

Ultimately, though, as developments in Britain show, "parent refusal can be tolerated (to a certain extent), but at some point the community becomes threatened", Marcuse says.

This points to the need to proceed carefully with vaccinations. One way to ease parental distress at the number of vaccines would be to space the jabs out more. Lainie Friedman Ross, professor of pediatrics and associate director of the MacLean Center for Clinical Medical Ethics at the University of Chicago, suggests that vaccines for diseases with little childhood risk, such as hepatitis B, could be delayed.

Diekema says that doctors may have to be more selective about which children get which vaccinations. "One of our responsibilities is to look carefully at newly developed vaccines and decide whether they should be recommended for all children," he says. "The more vaccines we decide should be administered, the more parents may object - no one likes to see children become pin cushions."

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