A Pakistani policeman was shot dead on Tuesday while protecting a polio vaccination team, police said, bringing the death toll in such attacks to 20 since December.
No one has claimed responsibility for the killings. But the umbrella Taliban faction last year banned polio vaccinations in the tribal region of Waziristan, alleging the campaign was a cover for espionage.
Rumours about vaccines being a plot to sterilise Muslims have also dogged efforts to tackle the highly infectious disease.
Tuesday's killing happened at Ghalla Dher on the outskirts of the northwestern town of Mardan, on the second day of a three-day local anti-polio campaign.
"The female vaccinators went inside a house to administer the polio drops. A police guard accompanying the team was waiting outside," Mardan district police chief Danishwar Khan told AFP.
"Two people came on a motorbike. They opened fire and shot the policeman dead," he added.
Pakistan, Afghanistan and Nigeria are the only countries where polio is endemic. Polio cases in Pakistan hit 198 in 2011, the highest figure for more than a decade and the most of any country in the world, according to the UN.
In January two polio vaccination workers, a policeman and seven members of a charity involved in polio vaccinations were shot dead in three separate incidents across the northwest.
In December gunmen killed nine health workers carrying out polio vaccinations in Karachi and the northwest.