The polio team was busy to administrate drops to children in the outskirts of Mardan, a main city some 120 northwest of Peshawar, when unidentified armed men opened indiscriminate firing, killing the policeman at the scene.
"No one from the polio team was hurt," a police officer said. The attackers fled after the incident.
No group claimed responsibility for the attack.
Taliban groups had banned polio drops in the tribal regions to protest against the US drones strikes.
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In December, the UN children's agency UNICEF and the World Health Organisation suspended the polio vaccination campaign in Pakistan after a series of attacks against workers.
Nearly 20 polio vaccination workers, including women, have been killed in attacks over the past three months.
In July, armed men also killed a Pakistani doctor working for the World Health Organization (WHO) on a national immunisation campaign.