The day before he relented and allowed Yahia to take the two drops of vaccine, he almost came to blows with the Unicef staff trying to inoculate the four-year-old against the disease, which can bring paralysis in a matter of hours.
Like thousands of parents in the northwest, Khan had heard and believed the rumours and conspiracy theories about the vaccine, which have helped the country maintain its unenviable status as one of only three nations in the world where polio is still endemic.
“I heard that the vaccine contains pig, that it’s haram (forbidden in Islam),” the 27-year-old Khan told AFP at his stall in Peshawar, surrounded by crates of fizzy drinks.
“Sermons from the mosque loudspeakers said it was an American conspiracy to damage our children.”
There have been 30 confirmed cases of polio in Pakistan this year according to the government, 22 of them in the tribal areas of the northwest, bordering Afghanistan.
Pakistan had a total of 91 polio cases last year, but the battle to convince people in the tribal areas, where education is limited and deeply conservative values hold sway, is a tough one.
Doctor Syed Irfan Ali Shah, 28, spent two years raising awareness among the local population and now heads the local Unicef team in Peshawar.
“We are welcomed because every member of my team is a local, well known in his neighbourhood,” he said.
“Trust is built up, and it is usually at the end of a number of visits that we manage to persuade the families.”
People in the area were already deeply distrustful of foreign intervention, and suspicions soared even further last year after the CIA used a hepatitis inoculation programme as cover to try to find Osama bin Laden.
The CIA used Pakistani doctor Shakil Afridi to set up a hepatitis drive in Abbottabad – where Bin Laden was later found and killed by US special forces – in the hope of obtaining DNA samples to identify the al Qaeda leader.
The episode made people all the more suspicious of vaccination, but campaign worker Zain Al Abedin said it was not the only reason people refused to give their children the drops.
“Massive illiteracy creates all sorts of fantasies. I’ve heard everything – the vaccine makes you sterile, it gives you AIDS, even that we urinated in it!” he said with a rueful smile.
Fighting between government troops and tribal militias in the northwest, as well the Taliban banning inoculations in protest at US drone strikes, have also hampered efforts to fight the disease.
In July officials said the problems were jeopardising more than 350,000 children in the tribal areas.
To improve matters, Unicef has tried to rally influential religious leaders to the cause.
In the poor Peshawar neighbourhood of Yakatoot, as naked children play in stagnant water, a team with a fatwa or religious ruling approving vaccination try to persuade a young father, Noor Zamin.
But his brother, a member of a religious group, steps in.
“I have made scientific studies on this. I cannot say what I discovered, but the vaccination is anti-Islamic,” he says, stroking his beard, before asking the aid workers to leave.
In a small mosque in the neighbouring area of Sadiqabad, the mufti Fidah refused to give a fatwa backing the vaccination.
“On one side, the United States and the Westerners bombard us with their drones, and on the other one they want to inoculate our children,” he said.
He was convinced “there is necessarily something bad” in the vaccine, but Shah the doctor says the cleric is softening his position, bit by bit.
Will he agree to the next vaccination campaign, in October? “Inshallah,” he says: if God wills it.