The gap between the number of vaccines administered in rich countries and by Covax is growing every single day, WHO's director general warned.
Dr Tedros Adhanom Ghebreyesus said any delay in supplying vaccines to poor countries will lead to a worsening of the pandemic, with the emergence of new variants that could evade existing vaccines.
He said the inequitable distribution of vaccines was not just a moral outrage.
"It is also economically and epidemiologically self-defeating."
The world will not be safe until everyone is safe, he said.
"The more transmission, the more variants. And the more variants that emerge, the more likely it is that they will evade vaccines. As long as the virus continues to circulate anywhere, people will continue to die.
"Ultimately it's not vaccines that save lives, it's vaccinations. The scale of what's needed in terms of the number of vaccinators, engagement with communities and ongoing monitoring, and health system strengthening is truly unprecedented."
Dr Tedros welcomed the Hope Consortium initiative led by the UAE on behalf of the WHO, saying he looked forward to working in the partnership in support of the Covax centre.
"We can only meet this challenge together with a co-ordinated, consistent and coherent approach," he said.
"Ultimately ending the pandemic is not just a test of science. It is a test of character."
Dr Tedros was speaking at an online conference organised by the Hope Consortium.
The World Immunisation and Logistics Summit, a two-day event that concludes on Tuesday, is discussing the global distribution and administration of vaccines to tackle the pandemic.
The Hope Consortium, which includes UAE national airline Etihad Airways, is an Abu Dhabi-led public-private partnership responsible for the mass distribution of Covid-19 vaccines.
More to follow.