The alliance, set up by IT magnate Bill Gates and funded by donor governments, international institutions and private philanthropists, also said that 213 million children will have been reached with GAVI-supported vaccines in the period 2000-2008.
"These numbers show the positive results of investment in human lives," said GAVI Executive Secretary Julien Lob-Levyt, urging donors to continue to support the programmes despite the current global economic crisis.
"Only through long-term predictable funding can we guarantee that poor countries are able to improve their immunisation programmes in order to save lives," he said.
GAVI made its latest projections on the basis of new data released by the World Health Organisation (WHO), which monitors the projected impact of the alliance's programmes in 76 developing countries.
In its latest progress report, the alliance said that the cumulative number of children benefiting from three doses of the so-called Hib vaccine (against a particularly deadly strain of meningitis - Haemophilus influenzae type B) is projected to rise to 41.7 million by the end of 2008, up from 28.2 million one year earlier.
Hib kills around 400,000 children under the age of five each year, mostly in the developing world.
Similarly, the hepatitis B vaccine is projected to reach a total 192.2 million children by the year's end, up from 155.7 million previously.
"With 2.5 million deaths averted through vaccination against hepatitis B alone, this is GAVI's single biggest success story," the alliance said.
GAVI said in March that hib meningitis had been virtually wiped out in Uganda thanks to its vaccination programmes.