A global vaccine group backed by Microsoft tycoon Bill Gates has asked donors for $3.7 billion to boost immunization among children in the world's poorest countries.

The Global Alliance for Vaccines and Immunization says it needs the money to fund the rollout of new drugs for 243 million children through 2015.

GAVI says the money would add to $3.1 billion already collected for vaccines against common childhood diseases as well as pneumonia and diarrhea, the two biggest killers of children in poor countries.

The Geneva-based group says the funds are needed despite pledges by vaccine makers to lower their prices for poor countries.

Companies like GlaxoSmithKline and Merck on Monday offered to cut the cost of some vaccines by up to 67 percent.