U.S. researchers on Sunday say they have developed a vaccine for tuberculosis that offers great protection in mice against the deadly disease, which kills 1.7 million people each year.

The scientists say injecting modified bacteria related to those which cause tuberculosis could offer unprecedented protect against the lung disease.

One in three people around the world is infected with tuberculosis, according to the World Health Organisation (WHO).

The only tuberculosis vaccine - the BCG jab - is not very effective. This vaccine has varying results of between 0 percent and 80 percent efficacy in different parts of the world.

Experiments on mice showed the injections could completely eliminate tuberculosis bacteria in some cases, according to reports in Nature Medicine.

Researchers led by William Jacobs, a professor at Albert Einstein College of Medicine in New York, has determined that the key to fighting off tuberculosis was to get a better understanding of how the bacterium that causes the disease finds its way around the human immune system.

Tuberculosis is caused by Mycobacterium tuberculosis, and WHO has said it is one of the top 10 leading causes of death.

Researcher worked with a closely related species called Mycobacterium smegmatis, that at high doses will be lethal to mice but does no harm to humans.They created a version of M. smegmatis lacking a set of genes, known as ESX-3, which allows the bug to evade host immunity.

The mice didn't get sick when they were injected with large doses of the altered bacteria, and without that missing genes, the modified bacterium wasn't able to past their immune system. The immune system fought off the infection using the same T-cells that a successful tuberculosis vaccine would need to activate.

The only problem is after removing the same set of ESX-3 genes from M. tuberculosis, it killed the bug and so it could no longer be manipulated to create the vaccine that has been long sought after.

Jacobs then created a hybrid of the two bacteria by inserting the ESX-3 genes from M. tuberculosis into the version of M. smegmatis from which the equivalent genes has been removed. The mice were still able to fight off the infection, and when they were exposed eight weeks later to high doses of the tuberculosis that kills humans the vaccinated mice lived nearly three times as long on average as a control group.

In mice infected with the tuberculosis bacteria, the ones that didn't get the vaccine died after 54 days on average. Those that were vaccinated with BCG lasted 65 days, while the mice immunized with the hybrid survived for 135 days.

This is something we've dreamed about for years, to be able to get longer protection and bactericidal immunity, Jacobs told the BBC.

He said in a statement that vaccinated animals that survived more than 200 days had livers that were completely clear of TB bacteria, and nobody has ever seen that before.

The study, published in Nature Medicine,noted that only a fifth of the mice were long-term survivors, which means that the vaccine must be improved.

We don't even know yet if it will work in humans, Jacobs said. But it is certainly a significant step.

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