The organisation said a steady supply is needed throughout the year as the poor countries do not have resources to use them all at once, the BBC reported.
UNICEF's vaccine lead Lily Caprani told BBC Newsnight that countries needed to vaccinate their own populations at the same time as the rest of the world.
"At some point, no doubt, we will need to vaccinate under-18s. But the priority at this moment has to be making sure that all of the vulnerable and priority groups around the world get vaccines.
"So we're saying countries like the UK and the G7 need to donate their doses to those low income countries now, while still vaccinating their populations at home," Caprani said.
UNICEF Goodwill Ambassadors Priyanka Chopra Jonas, David Beckham, Katy Perry, Orlando Bloom, Whoopi Goldberg, Angelique Kidjo and Liam Neeson have joined an extraordinary call by 28 high-profile UNICEF Ambassadors and Supporters demanding that G7 leaders commit to donating doses of Covid-19 vaccines to poorer countries now.
The open letter was published on Tuesday ahead of the three-day G7 Leaders' Summit (June 11-13) in Cornwall, the UK.
COVAX, the global initiative supporting poorer countries in gaining access to vaccines, is facing a shortfall of 190 million doses.
The letter urges the G7 leaders -- the UK, Canada, France, Germany, Italy, Japan and the US -- to commit to sharing a minimum of 20 per cent of Covid-19 vaccine dose supply between June and August, which would provide more than 150 million doses to COVAX.
"The world has spent a year and a half battling the Covid-19 pandemic, but the virus is still spreading in many countries and producing new variants with the potential to put us all back where we started," the letter read.
"This means more school closures, more healthcare disruptions, and greater economic fallout -- threatening the futures of families and children everywhere," it read.