In America, most children are vaccinated. A vaccine is also available in Canada. Australia is considering the idea.

In Europe, there is little vaccination as chickenpox is seen as fairly harmless.

Pharmaceutical company GlaxoSmithKline is developing a vaccine combining chickenpox and the controversial triple measles, mumps and rubella vaccine, but it will be at least two years before it is submitted for licensing approval.

After having chickenpox, people retain the virus within their sensory nerves.

It lies dormant there until a fall in immunity - which usually occurs after the age of 60 - allows it to flare up again as shingles (herpes zoster).

Children link

Around a quarter of people who have had chickenpox go on to suffer the condition, which manifests itself as a painful rash.

But adults living with children are less likely to develop shingles than those who do not.

This is because being exposed to children, and therefore chickenpox, acts like a booster vaccine against shingles.

The researchers from the Public Health Laboratory Service in London said if all children were vaccinated against chickenpox, adults who had had the disease would not be exposed to enough of the virus to prevent full-blown shingles later on.

They say that over the first 50 years, vaccinating a population the size of the US would save 5,000 children from dying of the complications of chickenpox.

But there would be 21m extra cases of shingles - and 5,000 people over 60 would die from complications associated with that condition.

John Edmunds, a member of the PHLS team, said: "Vaccination looks good in terms of costs and benefits if you just look at the economic effect of chickenpox, such as parents taking time off to look after children."

"But shingles has been ignored. If you include that, the costs and benefits may not be very good at all."

UK - 'no plans'

A spokeswoman for the PHLS said it was looking at what the impact might be of introducing a chickenpox vaccine in the UK.

As more evidence became available, it would be shared with the Joint Committee on Vaccination and Immunisation which advises the Department of Health, she said.

A Department of Health spokeswoman told BBC News Online: "There is no current plan to introduce universal immunisation against chickenpox, but we are aware of Dr Edmunds' work and will look at that."

In America, where vaccination was introduced in 1995, cases of chickenpox have fallen by up to 80%.

Michael Oxman, of the University of California at San Diego, said: "I started to support vaccination for chickenpox when I started seeing more deaths from Group A streptococcus infections."

These bacteria, which is the main complication of chickenpox in children, can cause toxic shock and the flesh-eating disease necrotising fasciitis, and are becoming more virulent.

Dr Oxman said there could be a surge in shingles, but suggested a solution could be to also vaccinate older people, something he is testing in an ongoing study of 40,000 Americans over 60.

He said even if that did work, it might be difficult to persuade some, particularly older men, to have their vaccinations.

The report is published in New Scientist and in the journal Vaccine.