The UN agency for Palestinian refugees reported the beginning of the polio vaccination in Syria's Yarmuk camp.
The once bustling residential and commercial district south of Damascus, home to both Syrians and Palestinians, had been largely sealed off by a tight army siege for months before a deal that allowed a trickle of food and medical aid in beginning last month.
At least 88 people have died of hunger and lack of medical care under the siege, according to one NGO, but UNRWA has now begun moving food parcels and medicine into Yarmuk.
"We are pleased to announce that UNRWA has secured the formal authorisation for the transfer of 10,000 polio vaccines to Yarmuk," UNRWA spokesman Chris Gunness said.
"This process has been completed without incident and the vaccination of thousands of children in the camp is now under way."
Gunness said food distributions that began on January 18, haltingly at first before picking up speed in recent days, continued on Wednesday.
He said 5,194 food parcels had been distributed to the camp since UNRWA gained access.
Some 1,500 people have left the camp under the deal, according to a Palestinian official, although they require approval to do so.
Once home to some 150,000 Palestinians, as well as Syrians, Yarmuk has been devastated by the fighting, and now just 18,000 civilians are thought to remain inside.
More than 130,000 people have been killed since the uprising against President Bashar al-Assad began in March 2011, and millions more have been displaced in what UN officials have described as the worst refugee crisis in two decades.