By Maggie Fox, Health and Science Editor

WASHINGTON, Nov 2 (Reuters) - A vaccine aimed at protecting children in developing countries from deadly tuberculosis may be killing and sickening some vulnerable infants infected with the AIDS virus, researchers said on Friday.

They said the Bacille Calmette-Guerin or BCG vaccine, which is made using a bovine version of tuberculosis, appeared to be causing serious infections in some babies and young children who are HIV-infected.

"One study found a 75-percent mortality rate in children with BCG disease, and 70 percent of those children were HIV-infected. Clearly, this is a problem in need of immediate attention," said Dr. Mark Cotton, a pediatrician and HIV researcher at Stellenbosch University in South Africa.

Cotton's findings are part of a report issued on Friday about the health emergency caused globally by the double whammy of HIV and TB.

The AIDS virus destroys the immune system, and tuberculosis has made a return globally because of this. Usually a latent infection, activated TB can kill quickly.

"Now the eye of the storm is in sub-Saharan Africa, where half of new TB cases are HIV co-infected, and where drug-resistant TB is silently spreading," said Veronica Miller, director of The Forum for Collaborative HIV Research, a global independent public-private group that includes researchers, patient advocates, and government and industry.

"It is here now. But the science and coordination needed to stop it are utterly insufficient."

The human immunodeficiency virus infects an estimated 40 million people globally. There is no cure and when untreated, it steadily destroys the immune system. Patients are vulnerable to a range of infections including TB.

BILLIONS INFECTED

TB infects one-third of the world's population. Without proper treatment, 90 percent of people infected with both die within months.

Usually, tuberculosis only becomes an active infection in one out of 10 people over a lifetime. But 10 percent of HIV patients who also have TB develop activated tuberculosis every year.

The BCG vaccine is given at birth in most developing countries. But because it uses a live microbe, in people with weakened immune systems it can itself cause disease.

"It is especially a problem where they have delayed access to diagnosis of HIV or delayed access to antiretroviral therapy," Cotton said in a telephone interview.

"It also is quite hard to diagnose it," he added. "We don't know how widespread it is across Africa."

Cotton said an estimated 400 per 100,000 HIV-infected infants in the Western Cape of South Africa had become sick from the BCG vaccine.

"The problem is the vaccine is usually given within the first few days of life," Cotton said. But babies are not tested for HIV infection until about 6 weeks of age, meaning many infants are unknowingly being given a vaccine that is dangerous for them.

Cotton said it might be possible to simply vaccinate children with BCG after it is known whether they are HIV-infected.

"But once you interfere with a program and make it a bit complicated, it can have repercussions as well, so it is a bit of a dilemma," he said.

The best result would be to have earlier diagnosis and treatment of HIV. Children infected with HIV can be given an antibiotic, isoniazid, to prevent TB infection, Cotton said.

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