The World Health Organisation (WHO), which helped run the testing programme, says that pneumonia, caused by pneumococcus bacteria, is the leading cause of death in children worldwide.

It claims approximately four million lives a year, predominantly in less developed countries.

Although the lung congestion has many causes, the most common is pneumococcus.

The bacterium can also cause meningitis, ear infections and sinusitis.

The trial involved 40,000 children in Soweto, and found that the new vaccine reduced the incidence of pneumonia by more than 20%.

In addition, the vaccine reduced the incidence of "invasive pneumococcal disease" - where bacteria can be found in the bloodstream - by more than 80%.

The invasive form of the disease was reduced in incidence even in HIV-infected children.

Pneumococcal disease is one of the predatory illnesses most likely to kill HIV-positive children in developing countries.

Thousands saved

Dr Keith Klugman, from Emory School of Medicine in the US, which helped develop the vaccine, said: "We could potentially save over 500,000 lives each year in the developing world.

"In addition, no vaccine has previously been documented to prevent pneumococcal disease in HIV-infected children, and our study showed a 50% reduction in this group.

"In an era when there is little to offer children with HIV, we can clearly reduce invasive disease."

The vaccine targets seven strains of pneumococcal disease, and has been well tested in the western world, but not previously in developing countries.

The South African version included two strains prevalent in the country.

Two groups of infants were involved in the study - one was given the jab, and another a placebo containing no active ingredients.