Human trials are being planned for a new tuberculosis vaccine that's designed to better protect HIV-positive people from TB. People infected with the AIDS virus (HIV) are more susceptible to TB because they have much weaker immune systems. As a result, health officials say both diseases are now closely linked and must be addressed at the same time.

Dr. Marcus Horwitz is professor of medicine, microbiology and molecular genetics at the UCLA David Geffen School of Medicine. From Los Angeles, he spoke to VOA English to Africa Service reporter Joe De Capua about why a new vaccine against tuberculosis is needed.

"A new TB vaccine is needed for all people, not just HIV-positive people, because the current vaccine, which is called BCG, is not very effective. It seems to prevent tuberculosis pretty well in children, but, particularly in adults, it's only at most about 50 percent effective in various clinical trials. So, there's a great need for a vaccine that's much more effective than that. And that would be true for all people," he says.

He explains why a TB vaccine specifically targeted for HIV-positive people was engineered. "The current vaccine is not recommended for HIV-positive people and that's because it can harm them. It can multiply uncontrolled in some of these people and cause a very severe disease and even death?. It (new vaccine) could be used by anyone and it is more potent than the current vaccine, but it was designed with the HIV-positive person in mind," he says.

HIV-positive people could be more at risk of developing TB if they had been given the BCG vaccine, according to Horwitz. "First of all, HIV is a progressive disease. Overall, patients with HIV are much, much more susceptible to tuberculosis than persons who have a normal immune system. Patients with AIDS have several hundred-fold-increased susceptibility to developing tuberculosis. The problem is that if someone, who is HIV positive, is not treated and eventually their white blood cells decline, then they become very susceptible to the vaccine, BCG, causing illness," he says.

Asked how the BCG TB vaccine could be responsible for an HIV person developing tuberculosis, he says, "BCG is a live vaccine. It's a live bacterial vaccine and it will grow unchecked in people if the immune system is not there to counter it."

However, the new vaccine candidate prevents the bacteria from uncontrolled reproduction. "We designed a vaccine that would only multiply a few times?and the idea was simply this, that you need to multiply a few times in order to induce a strong immune response to protect against tuberculosis. But it turns out you only need to multiply a relatively few times to do that. So, we designed a vaccine that was able to multiply two, three, four or five times in the host and then stop, " he says.

He says that was done by limiting the amount of iron available that bacteria need to reproduce.

Dr. Horwitz says, with some variation, the new vaccine should be effective against the new tougher strains of TB, such as MDR and XDR. The human clinical trials, however, will take years, with research being done in areas where there are many cases of TB.