The BCG vaccine, commonly given to schoolchildren in the UK, is more than 70 per cent effective in this country - but has virtually no effect on TB in South India.

The BCG vaccine works by using the germ which causes tuberculosis in cows, but is harmless to humans.

However, it is close enough to the harmful human tuberculosis bacterium to helps the body prepare for an attack by the real thing.

In a study published in Science magazine, Californian gene expert Dr Marcel Behr has looked in detail at the complete genetic makeup of both the cow tubercolosis, and the human version - and found around 100 differences, or "deletions".

Genes hold key to improving vaccine

Dr Behr believes that a more effective version of the vaccine can be produced by "improving" the genetic structure of the cow tuberculosis bacteria to make it more like human tuberculosis.

'Vaccine might fail if kept wrongly'

Prof Douglas Young, of Imperial College School of Medicine, agreed: "Although there is as yet no evidence to confirm the hypothesis that these deletions are responsible for changes in vaccine efficacy, the results provide a rational starting point for attempts to generate - or regenerate - a better BCG."

He described advances in genetic technology as offering an "exciting new era" in vaccine development.

However, another TB expert who has worked extensively in TB-ridden third world countries, said that the vaccine might be failing there for different reasons.

'Improvements may be waste of effort'

Dr John Lunn, a retired consultant in occupational medicine, said that storing the prepared vaccine in hot conditions, or injecting it with the wrong device might render it useless.

He said: "It doesn't matter how effective a BCG vaccine is, it won't be worth anything if it isn't handled properly.

Many third world clinics are trying to use old-fashioned re-usable BCG injectors on babies or younger children, reducing their efficiency.

A recent study in South Africa, he said, found that the vaccine had had virtually no effect on resistance to tuberculous meningitis, indicating that the vaccine had not "taken" in the body.