The World Health Organization (WHO) warned on Monday against a widening gap between numbers of coronavirus vaccines in wealthy countries and those distributed to poorer nations through the global COVAX initiative.
"The gap between the number of vaccines administered in rich countries, and the number of vaccines administered through COVAX is growing every single day," UN health agency chief Tedros Adhanom Ghebreyesus said.
"The unequitable distribution of vaccines is not just a moral outrage, it's also economically and epidemiologically self-defeating," Tedros added, during a virtual conference hosted by the United Arab Emirates to address global immunization.
"As long as the virus continues to circulate anywhere, people will continue to die, trade and travel will continue to be disrupted, and the economic recovery will be further delayed."
It follows criticism by UN Secretary-General Antonio Guterres of nations creating a "stockpile" of COVID-19 vaccines, calling on them to share to help end the pandemic.
Health officials have rolled out more than 510 million coronavirus vaccine doses around the world.
Tedros had called for all countries to begin vaccinating within the first 100 days of the year, but as that deadline nears, 36 countries are still to receive a single dose.
Sixteen of those are scheduled to receive their first doses through COVAX within the next two weeks, but the other 20 countries are expected to miss out.
The UN's children's agency on Monday urged wealthier countries to ensure equitable distribution of vaccines, adding that $510 million is needed to support delivery around the world.
"We need help," UNICEF chief Henrietta Fore said at the virtual conference. "We need vaccine manufacturers to prioritize COVAX and work to secure regulatory approval for fast, fair and affordable distribution. We need wealthier nations to donate extra doses through COVAX."
The COVAX initiative was expected to deliver some 238 million doses around the world by the end of May, and has so far shipped more than 32 million doses.