The program also aims to increase vaccine production capacity and encourage companies to develop vaccines for diseases common in the poorest countries.
Prevnar was launched in Western countries in 2009. Pfizer made the first supply agreement for it under the program in 2010, and a second one in 2011. Those deals, running through 2023, covered 480 million doses.
The alliance has estimated the pneumococcal vaccines could save up to 1.5 million lives by 2020.
"More than 10 million children have been reached with GAVI-supported pneumococcal vaccines in 29 countries since 2010. We expect to reach children in more than 50 countries with this lifesaving vaccine by 2015," Dr. Seth Berkley, CEO of the GAVI Alliance, said in a statement, adding that the goal is to make the program sustainable in the long term, partly by securing very low vaccine prices.
The latest deal gives Pfizer $3.40 a dose for Prevnar this year, then $3.30 per dose through 2025. For about the first 20 percent of doses, Pfizer gets an extra $3.50, from the $2.8 billion pledged by the charities and wealthy governments.
"Strong vaccination programs are a cornerstone of economic development -- a simple intervention that has dramatic short- and long-term impact on health," Susan Silbermann, Pfizer's president of vaccines, said in a statement.
New York-based Pfizer is the world's second-biggest drugmaker, selling medicines including Viagra and pain relievers Lyrica and Celebrex. Prevnar is the top-selling vaccine ever, with annual sales just more than $4 billion.