A vaccine used to combat tuberculosis may have outlived its usefulness, a new study suggests.

An international team of researchers, including Dr. Marcel Behr of Montreal's McGill University, has found that a bacterium commonly used for the vaccine may not be as effective as it once was.

"We've determined that this bacteria, which is given as a vaccine two million times a week around the world, is highly evolved from when it was first used in 1921," said Behr.

"Consequently, we're not really sure if it's as effective as it's been in the past or as it could be."

The team mapped and analyzed the genetic code of bacillus Calmette-Guérin (BCG), a derivative of Mycobacterium bovis -- which is part of the World Health Organization's Expanded Program on Immunization.

The research indicates that the vaccine has been weakened because of genetic factors.

Previous trials dating back to the 1960s have questioned the effectiveness of BCG but Behr said the new research "puts a punctuation mark" on the urgency for a more effective vaccine.

The research has been presented to the WHO. The organization has named global tuberculosis control as one of the top priorities of 2007.

Behr conducted the study with researchers from France's Institut Pasteur as well as genetic labs in the United Kingdom.

The findings have been published in the Proceedings of the National Academy of Sciences (PNAS) Online.

TB kills nearly 2 million people every year. It commonly affects the lungs but can also affect the central nervous system, and the lymphatic, circulatory and genitourinary systems as well as bones, joints and even the skin.