The latest campaign in Khyber Pakhtunkhwa was launched Monday to give oral drops to children who had missed it the first time around.
Pakistan is one of only three countries where the crippling disease is endemic. The virus usually infects children living in unsanitary conditions. It attacks the nerves and can kill or paralyze. As many as 56 polio cases were reported in Pakistan during 2012, down from 190 the previous year, according to the United Nations.
Most of the new cases in Pakistan were in the northwest, where the presence of militants makes it difficult to reach children for vaccination.
Pakistan is also struggling to maintain control of its southern province of Baluchistan.
On Tuesday, gunmen driving in a car opened fire on two police constables who were patrolling on motorcycle in a neighborhood of the provincial capital, Quetta, said Nadir Khan, a local police chief. Both the police officers were killed, he said.
There was no immediate claim of responsibility, but similar attacks against security forces are believed to be the work of Baluch nationalists who have been pushing for a greater say in use of the province's resources.
Also Tuesday, gunmen riding on two motorcycles opened fire on a police station in the country's largest city, Karachi, killing one police constable and injuring two others, said senior police officer Alim Jaffery. He said the gunmen escaped.
Associated Press writers Adil Jawad in Karachi, Pakistan, Abdul Sattar in Quetta, Pakistan and Zarar Khan in Islamabad contributed.