According to a report published on Thursday by the UN children's agency, major gains made in vaccinating the world's children during the 1980s have levelled off.
Donor nations must understand that progress to bring immunisations to those remaining will take renewed efforts and more cash, said Dr Peter Salama, Unicef's chief of immunisations.
"Everybody thought that we were progressing so well that we would just progress continually," Salama said. "But in fact that didn't happen."
About 130 million children are born each year, and since 1990, about 70% have received the most vital immunisations. That's up from some 20% under the age of one in 1980.
But since then, there has not been great progress in reaching the final 20 or 30% who need help - mainly in poor countries - and those are the places that need the most urgent attention, Unicef said.
Salama added about $1 billion is now being spent on childhood immunisation and about $1 billion more is needed to reach a goal set in 2002 to bring vaccines to at least 90% of children under the age of one around the world by 2010.
Immunisation programmes in
several countries have shrunk
That figure will rise to about $6 billion as new vaccines come to market for killers such as rotavirus, which causes acute diarrhoea, and pneumococcal disease, which leads to pneumonia.
A Unicef report highlighted the sharp divide between vaccinations in rich and poor nations. In 2003, 90% of children in industrialised nations had proper immunisations. But coverage rates in West and Central Africa are just 52%, the report said.
It said that overall, 103 countries have 90% protection rate against measles, while 16 are likely to achieve that rate by 2010. Another 55 need improvements, while 16 must reverse declining immunisation rates.
"There is, in my view, nothing more important than saving a child's life, and we need to strengthen our advocacy to ensure that the funds are available to meet those goals," Salama said.