Government officials in Peshawar said that they believe a Taliban faction in Mohmand, a tribal area near Peshawar, was behind at least some of the shootings. Still, the Pakistani Taliban have been uncharacteristically silent about the attacks, with no official claims of responsibility. In staying quiet, the militants may be trying to blunt any public backlash like the huge demonstrations over the attack on Ms. Yousafzai.

Female polio workers here are easy targets. They wear no uniforms but are readily recognizable, with clipboards and refrigerated vaccine boxes, walking door to door. They work in pairs — including at least one woman — and are paid just over $2.50 a day. Most days one team can vaccinate 150 to 200 children.

Faced with suspicious or recalcitrant parents, their only weapon is reassurance: a gentle pat on the hand, a shared cup of tea, an offer to seek religious assurances from a pro-vaccine cleric. “The whole program is dependent on them,” said Mr. Shah, in Peshawar. “If they do good work, and talk well to the parents, then they will vaccinate the children.”

That has happened with increasing frequency in Pakistan over the past year. A concerted immunization drive, involving up to 225,000 vaccination workers, drove the number of newly infected polio victims down to 52. Several high-profile groups shouldered the program forward — at the global level, donors like the Bill and Melinda Gates Foundation, the United Nations and Rotary International; and at the national level, President Asif Ali Zardari and his daughter Aseefa, who have made polio eradication a “personal mission.”

On a global scale, setbacks are not unusual in polio vaccination campaigns, which, by dint of their massive scale and need to reach deep inside conservative societies, end up grappling with more than just medical challenges. In other campaigns in Africa and South Asia, vaccinators have grappled with natural disaster, virulent opposition from conservative clerics and sudden outbreaks of mysterious strains of the disease.

Their natural first reaction is frustration. But then, each time, vaccinators have optimistically predicted that, with enough donations and a redoubled effort, they would get the situation under control.