A new vaccine against pneumonia is being rolled out in Africa which estimates suggest could save more than half a million lives a year globally.
The Gavi Alliance, a global health partnership of public and private sectors for immunisation, says 19 countries will get the jab at first.
Kenyan children have begun receiving it and Sierra Leone, Yemen, Honduras and Guyana will follow this year.
Infants in Nicaragua also started receiving the vaccine a few weeks ago.
Many more countries could benefit if the funding becomes available.
Gavi says it needs an extra £500m ($800m) annually for the next five years to meet a shortfall in immunisation for existing and new vaccines.
The pneumonia vaccine protects against pneumococcal disease, the leading cause of severe pneumonia in children. It also guards against a form of meningitis and blood poisoning.
Pneumonia kills more children than any other illness, claiming around 1.7 million lives every year.
At the Langata health centre in Nairobi, scores of mothers brought their babies along for the first of three injections.
Beatrice Aching's son Wesley died from pneumonia in November. She brought her three-month-old daughter Tamara to be immunised. She said: "My son's death happened very suddenly. Wesley got sick in the morning and by evening he had died in hospital - I don't want that to happen to Tamara."
Leah Otieno's nine-month-old son Emmanuel got pneumonia before Christmas but recovered after antibiotic treatment - she says she is delighted to get him protected.
The charity Save the Children has launched a report, No Child Born to Die, which highlights the potential funding shortfall for global immunisation.
The report also says there is a critical shortage of 3.5 million health workers in poor countries, without whom millions of children will face illness and early death.
"Too many children are dying every day of vaccine-preventable illnesses and from the lack of basic healthcare," said Catherine Fitzgibbon from Save the Children. "The money needed for basic immunisation is in doubt, let alone for this effective new vaccine against pneumonia."
In June 2011, the UK government is hosting a meeting of Gavi in London which will be attended by world leaders. The UK provides a quarter of all Gavi's funding - more than any other nation.
Save the Children says it will be campaigning for rich nations to increase support for global immunisation, and for the pharmaceutical industry to lower the price of vaccines.
The pneumococcal vaccine costs £2.20 ($3.50) in Africa compared to £38 in Europe as a result of a deal between Gavi and two manufacturers: Pfizer and GSK. The roll-out in the developing world comes just a year after the same vaccine was introduced in the United States.
GSK said the discounted price is only fractionally above the cost of production. A spokesman said the vaccine takes a year to produce and is the most technically sophisticated of all its vaccines.
A second vaccine against rotavirus - the main cause of serious diarrhoea - is also being ready to be rolled out. But this, too, is far more expensive than the basic childhood vaccines against diseases like measles, whooping cough and polio.
Pneumonia and diarrhoea account for a third of all deaths in young children in the developing world. Gavi and Save the Children say a comprehensive roll-out of the pneumococcal and rotavirus vaccines could potentially prevent more than one million deaths annually.