"What we see now is that the A(H1N1) virus is beginning to penetrate into some of the poorest communities in the world," said Julie Hall, an expert from the UN's World Health Organisation.
"We are anticipating that we may well see a different pattern of impact once this virus starts to take off and those explosive outbreaks occur in poorer communities," she told a news conference
David Nabarro, UN senior influenza coordinator, said both manufacturers and rich countries were ready to make vaccines available for developing nations, but warned that the supply would still be inadequate.
"We are aware that there will only be enough for a small percentage of the population of developing countries," he said.
"The challenge during the next few weeks is... to ensure that adequate vaccines reach health workers and essential personnel in developing countries in time to help them as the next waves of the pandemic reach them," he added.
Last week, the WHO said pharmaceutical firms can produce only three billion doses of swine flu vaccines a year, covering less than half of the global population.
Nabarro also called for cash donations to help low-income countries prepare for the pandemic by raising awareness and improving their health services infrastructure.
At least 3,917 people have died from the A(H1N1) virus since it was uncovered in April, with most of the fatalities in the Americas region, according to the WHO.