BCG is not as effective as more modern vaccines like MMR, which gives 99% protection after two doses

Public Health Service Laboratory spokesman

BCG is a weakened strain of Mycobacterium bovis, the bacterium that gives cattle thieir version of TB.

While the vaccine has saved millions of lives, it still fails to prevent TB in as many as 80% of the people who get the shots.

"Now we have a good explanation for the failure of BCG and have found a way round the problem," said Peter Andersen, head of the research team which made the discovery in Denmark.

He and his colleagues believe the failure of BCG is due to weaker relatives of Mycobacterium tuberculosis, the bug that causes TB in people.

These strains accidentally sabotage the vaccine by getting to people first, via water and food in hot countries.

BCG immunity

Mr Andersen said: "Essentially they immunise people against BCG."

In an article in New Scientist, Mr Andersen says they prevent the BCG vaccine from multiplying inside the body and do not trigger enough immunity to protect against TB.

This theory explains why BCG will protect infants if they are injected just after birth, before environmental mycobacteria can get in the way.

Mr Andersen proved the link by infecting mice with three related strains of mycobacteria, all of them isolated from patients and soil in parts of Malawi, where BCG does not work.

In each case, subsequent BCG shots failed to stop the mice getting TB.

Paul Fine, professor of communicable disease epidemiology at the London School of Hygiene and Tropical Medicine, says the paper "comes up with the best evidence yet" that prior exposure to related bacteria nullifies BCG.

New vaccines

A spokesman for the Public Health Laboratory Service said the BCG vaccine has had a powerful effect on TB.

He said: "To suggest it is ineffective would be misleading.

"BCG is not as effective as more modern vaccines like MMR, which gives 99% protection after two doses.

"But it's a tool in the armoury, but not the only device to control the disease.

"We need early diagnosis, rapid treatment and good screening facilities."

There is good news that new vaccines made from fragments of dead TB bacteria did protect the pre-infected mice from TB.

Unlike live TB vaccines, these do not need to multiply in the body to be effective.

Mr Andersen said: "There are many such vaccines in the pipeline."

Only one has so far reached the stage of clinical trials.

It is now hoped new vaccines might provide immunity for adolescents whose BCG protection has either worn off or never took hold in the first place.

There was a record rise in new TB cases last year in the UK, with almost 7,000 people diagnosed with the condition.