Nearly half a million people have fled a military operation against Taliban strongholds in North Waziristan, a hotspot for the crippling disease in Pakistan.
Children in the tribal district have not been vaccinated since Taliban and local warlords banned health teams from giving out drops in June 2012.
Tens of thousands of families have fled to the town of Bannu, close to North Waziristan, while hundreds more have moved further afield to Lakki Marwat, Karak and Dera Ismail Khan towns, since the offensive began in mid-June.
Officials have begun a vaccination campaign in Bannu and three other districts adjacent to North Waziristan, vaccinating both resident families and newcomers fleeing the offensive.
"We are vaccinating both local and displaced children, the target is to vaccinate more than 200,000 children," doctor Akbar Jan, a senior health official in Bannu, told AFP.
The campaign in areas adjoining North Waziristan began -- unannounced -- on Monday.
"Displaced persons were a threat to the host communities, now we have the opportunity to vaccinate both host community and displaced families," Jan said.
More than 50 cases of polio have been detected so far this year in militant-infested North Waziristan, out of 82 cases across the country -- and 103 worldwide.
A World Health Organization (WHO) official in Bannu told AFP the campaign would continue one day a week during the fasting month of Ramadan, which begins at the weekend.
- Vaccine rumours spurned -
Pakistan is one of only three countries, along with Afghanistan and Nigeria, where polio remains endemic, and efforts to eradicate it have been badly hit by rumours about the vaccine.
Various outlandish claims have circulated about the drops -- that they contain pork, or cause infertility or AIDS.
But health workers giving out the vaccine in a narrow street in Bannu's Tanchi bazaar area said they had encountered little resistance.
"This is a house-to-house campaign, our team has vaccinated 300 children in two days," Shumaila Khan told AFP.
"So far no family has refused to vaccinate their kids. There were many who were reluctant at first but later convinced."
Many parents had heard the rumours about the vaccine, Khan said.
"They said the Taliban told them it was an American conspiracy to disable their children, to make them infertile and to decrease the Muslim population," she said.
Sharif Zaman, a 35-year-old teacher sheltering in a school with 10 other families after fleeing North Waziristan, recalled the militants' propaganda.
"They used to tell us your children will suffer epilepsy and would become abnormal," the father-of-five told AFP.
Zaman had five kids who were all were vaccinated at a check post.
Naimatullah Khan, who was running a restaurant in Mir Ali, said militants used to distribute leaflets saying anti-polio drops were perilous.
"They used to threaten the whole population that any one whose child had polio drops would be slaughtered," he said.
"They used to say we will cut your throat with a dagger."
At the start of May, WHO declared a global "public health emergency" after new polio cases began surfacing and spreading across borders from countries including Pakistan.
Pakistan imposed new travel guidelines after the WHO move, requiring all citizens and long-term residents to have a polio vaccination certificate to travel abroad.