It was the latest of several attacks on Pakistani efforts to eradicate the deadly disease, found in only three countries in the world. Militant extremists view the vaccination campaigns as Western-backed plots to gain intelligence in sensitive areas and have frequently targeted the medical staff and those protecting polio teams.
No polio workers were wounded in Tuesday's attack in the Mardan district of Khyber Pakhtunkhwa province, said police officer Fazal Wahid. At least two attackers were hiding in a field near a narrow road as the polio workers walked by on their way to visit houses in the area, said Mardan Police Chief Inam Jan.
"The polio workers were going door-to-door and one police officer was protecting them when the gunmen suddenly attacked them near an open area and fled," Jan said, adding that the police were searching for the attackers but that so far no one had been arrested.
No one immediately claimed responsibility for the attack and it wasn't immediately known whether the police officer was targeted because he was protecting the polio team or for some other reason.
Janbaz Afridi, a senior health official, said the polio vaccination campaign continued in various parts of the province Tuesday despite the killing. "We have taken best possible steps for the safety of polio teams," he said.
In 2012, humanitarian workers, including those working to prevent the polio spread, were repeatedly targeted. According to UN figures, 19 humanitarian workers were killed last year in Pakistan. Of those deaths, 11 were related to polio, including a rash of shootings in December when nine polio workers were killed across Pakistan.
In an effort to protect people administering the vaccine, the government has increasingly sent police officers into the field along with the vaccinator. But they have come under attack as well.
On Jan. 29, gunmen riding on a motorcycle shot and killed a police officer protecting polio workers in the Swabi district in Khyber Pakhtunkhwa province.
Mazhar Nisar, a senior official working with the Prime Minister's polio monitoring cell, said at least 11 members of polio teams have been killed in various parts of Pakistan since December.
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.