A vaccine could cut tuberculosis cases among HIV-positive Africans by almost two-fifths, a US study suggests.

The lung infection is the most common cause of death among HIV patients in the continent.

Journal Aids reports that Dartmouth Medical School research involving 2,000 people found significantly fewer TB cases in vaccinated patients.

An expert said the jab could be a cheaper option for countries struggling to find money for extra anti-HIV drugs.

HIV patients are particularly vulnerable to TB because their immune systems are compromised.

The vaccine works by boosting the immune responses of patients who have already been given the BCG vaccine earlier in life.

In itself, the BCG jab may offer some protection against TB, but this is far from certain, and protection may only last a few years after immunisation.

The researchers from Dartmouth Medical School in the US tested it among 2,000 HIV positive patients in Tanzania over a seven-year period.

The number of confirmed TB cases was 39% lower in the vaccinated group.

First vaccine

Professor Ford von Reyn, who led the study, said it was a "significant milestone".

One theory now suggests that patients could be given the booster jab as soon as they are diagnosed with HIV, before antiretroviral drugs are needed.

Alvaro Bermejo, executive director at the International HIV/Aids Alliance, said that the other way of fighting TB in HIV patients might be to give them antiretrovirals earlier, an expensive option compared with a vaccination programme.

He said: "This is a very important finding - it is the first time we are going to have a vaccine which is influential in preventing opportunistic infections in HIV patients.

"TB is a massive problem - a third of people living with HIV in Africa are infected with it.

"The reduction of 39% seen in Tanzania, although not fabulous, is a good result."