* Money would fund GAVI's programs from 2016 to 2020
* GAVI works with drugmakers to cut vaccine costs for poor
* 1.5 mln children a year die of vaccine-preventable disease (Updates with pledge from European Union, price freeze from GSK)
By Kate Kelland
LONDON, May 20 (Reuters) - The GAVI global vaccines alliance appealed on Tuesday for $7.5 billion to help immunize another 300 million children against life-threatening diseases between 2016 and 2020 and save up to 6 million more lives.
GAVI said the additional investments, which it hopes to get mainly from global health philanthropists and the governments of developed nations, could double the number of lives saved through GAVI-supported vaccines to an estimated 12 million.
"We are faced with an historic opportunity to support countries to build sustainable immunization programs that will protect entire generations of children," the group's chairman, Dagfinn Hoybraten, said in a statement.
"The investments we all make now can ensure the equivalent of two children every second will be reached with GAVI-supported vaccines for five years and secure the future health and economic prosperity of all our children in years to come."
GAVI, which is backed by the Bill & Melinda Gates Foundation, the World Health Organization (WHO), the World Bank, UNICEF, donor governments and others, funds immunization programs for nations that cannot afford standard prices.
The group targets common but deadly diseases such as pneumonia, diarrhea and cervical cancer and says it has already saved around 6 million lives since its launch in 2000.
Seth Berkley, GAVI's chief executive, told Reuters the alliance's target of an extra $7.5 billion would be added to $2 billion already in hand for 2016-2020.
This is around 15 percent more than GAVI has for the current five-year period, he said, and is necessary because around 1.5 million children die each year of vaccine-preventable diseases.
The European Union announced in response on Tuesday that it would pledge 175 million euros ($240 million) in funding for the alliance for 2014-2020.
Berkley said such programs would produce gains worth $80 billion to $100 billion, by cutting the cost of treating illness and by keeping people well and productive into adulthood.
GAVI uses its private and government donors' backing to negotiate with pharmaceutical firms such as GlaxoSmithKline , Merck and Pfizer to bring down vaccine prices for the poor. It then works with partners to bulk-buy and deliver vaccines to countries whose populations need them most.
The group says its influence on the vaccines market so far has led to a 37 percent decrease in the cost for a GAVI-eligible country to vaccinate a child with pentavalent, pneumococcal and rotavirus vaccines since 2010.
GSK, which sells several of its vaccines - including rotavirus, pneumococcal disease and cervical cancer shots - at reduced prices through GAVI to poor countries, said on Tuesday it would continue to freeze those prices for a further five years for countries who "graduate" from GAVI support.
By 2020, 22 countries with growing economies will become too wealthy to be eligible for GAVI's help, allowing GAVI to focus on the poorest countries and give developing economy governments responsibility for immunization. ($1 = 0.7289 Euros) (Editing by Ruth Pitchford)