Some militant groups in Pakistan oppose the vaccination campaign, accusing health workers of acting as spies for the United States or the Pakistani government. They are also angered since it became known that a Pakistani doctor helped in the U.S. hunt for Osama bin Laden.
The physician, Shakil Afridi, ran a hepatitis vaccination campaign on behalf of the CIA to collect blood samples from bin Laden's family at a compound in northwestern city of Abbottabad, where U.S. commandos killed the al-Qaida leader in May 2011.
The samples were intended to help the U.S. match the family's DNA to verify bin Laden's presence there. In the recently released film about the search for bin Laden, "Zero Dark Thirty," a short scene shows a man going to vaccinate people at the compound where bin Laden was hiding.
The campaign however is portrayed in the movie as an anti-polio campaign, not anti-hepatitis.
The campaigns are made more complicated by the fact that many Pakistani residents are also suspicious of the repeated vaccination efforts going on across the country and fear the vaccines are intended to make Muslim children sterile.
Pakistan is one of the few remaining countries, along with Afghanistan and Nigeria, where polio is rampant. As many as 56 polio cases were reported in Pakistan during 2012, down from 190 in 2011. Most of the new cases in Pakistan were in the northwest, where the presence of militants makes it difficult to reach children for vaccination.
The virus usually infects children living in unsanitary conditions. It attacks the nerves and can kill or paralyze.
Associated Press writer Rebecca Santana in Islamabad contributed to this report.