Donors have pledged $US7.5 billion to extend a child vaccination drive in developing countries.

Countries and private donors have pledged $US7.5 billion ($A9.45 billion) to help immunise 300 million more children in developing countries over the next five years, a major vaccine alliance has announced.

The Global Alliance for Vaccines and Immunisation (GAVI) announced on Tuesday it had just slightly exceeded its "bold" request for funding for 2016 to 2020 at a two-day pledging conference in Berlin.

The funding will help the alliance, created in 2000 as an international public-private partnership, support developing countries in vaccinating an additional 300 million children, saving up to six million lives, it said.

"We have never had a replenishment this size before in GAVI's history. It's record breaking. It was a very bold ask we presented to the world leaders but also a very compelling case," GAVI board chairman Dagfinn Hoybraten told reporters.

He said that some current GAVI donors had increased their support "substantially" and that four countries, including China - previously a GAVI funding recipient - had become new donors.

German Chancellor Angela Merkel told donors earlier on Tuesday that there had been much improvement thanks to the work of GAVI but that challenges remained, highlighting that the Ebola epidemic in Africa had revealed the weakness in some countries' health systems.

She said public health would be a priority for Germany's G7 presidency this year.