The GAVI vaccine alliance, which is backed by software billionaire Bill Gates, said Thursday that it needed an extra 4.3 billion dollars to save millions of lives in poor countries over the next five years.

The Global Alliance on Vaccines Initiative says it has helped boost immunisation levels since 2000 by funding new vaccination programmes for 256 million children and avoiding 5.4 million premature deaths.

But GAVI officials said here that they would try to convince governments, private foundations and other donors to expand the existing 2.7 billion dollar vaccine programme until 2015 at a meeting next week.

Communications director Jeffrey Rowlands said the target was to save 4.2 million lives by 2015.

"GAVI needs about 4.3 billion dollars for 2010 to 2015, the majority for pneumococcal and rotavirus vaccines," he added.

So far GAVI and its partners have focused on tackling diseases such as diptheria, hepatitis B, tetanus, whooping cough and yellow fever.

The bulk of the expansion would be devoted to recently approved vaccines to prevent the most severe cases of pneumonia and diarrhoea-inducing rotavirus, which kill an estimated two million children in developing countries every year.

Judith Kallenberg, senior manager for donor relations, said the alliance and its pooled resources had brought new vaccines far more swiftly than before into poor countries.

"We would like to do the same thing with pneumococcal and rotavirus vaccines, to prevent the two biggest killers of children," she said.

"The objective is not only rallying existing troops, but engaging others," Kallenberg added, arguing that it was also a cost effective way of boosting economic development.

The GAVI alliance brings together governments, the World Health Organisation, UN Children's Fund UNICEF, the World Bank, non-governmental organisations and private donors such as the Bill and Melinda Gates Foundation.

Part of its success has been in driving down the cost of vaccines by striking bulk deals with pharmaceutical companies, and investing in vaccines that would have been largely neglected because they are most needed in poor countries.