ISLAMABAD, Pakistan -- A roadside bomb killed two polio workers in northwestern Pakistan on Thursday, in the third such attack this week on workers struggling to immunize children against the crippling disease.
The explosion struck as the two workers, with a United Nations-backed campaign, were traveling by motorcycle near Parachinar in the Kurram district, near the border with Afghanistan.
It was the first such attack on health workers in that area, said a senior local official speaking by phone on the condition of anonymity, offering further evidence that a Taliban-led campaign of violence and intimidation against polio workers is spreading across northwestern Pakistan.
Despite an internationally supported campaign to halt polio in Pakistan, infection rates have soared across the country in the past year, coinciding with a wave of militant attacks against the poorly protected workers at the heart of the effort. Some militants accuse polio workers of using vaccination campaigns as a cover to spy on behalf of the United States -- a claim that has been fueled by the revelation that the C.I.A. used a vaccination drive as cover for the effort to find Osama bin Laden in Abbottabad in early 2011.
The violence has badly affected vaccination efforts, which involve tens of thousands of health workers who repeatedly administer cheap oral vaccines to children under 5.
Nine polio workers were killed in a string of attacks across the country in December. On Tuesday, people suspected of being militants fatally shot a police officer who had been escorting female polio workers in the Swabi district, 50 miles east of Peshawar, the capital of Khyber-Pakhtunkhwa Province.
The shooting prompted officials to suspend the campaign in Swabi, but they pressed ahead in other parts of Khyber-Pakhtunkhwa and the adjoining tribal belt. Another polio worker was wounded by a man with an ax in an unrelated episode on Tuesday.
It was unclear, however, whether the latest killings were directly related to the polio campaign. Kurram district has a history of violence between Shiite and Sunni Muslims, and the local official said it was unclear whether the explosion targeted the two workers for their links to the polio campaign or for their religious affiliation.
Violence against polio workers is not confined to the turbulent northwest. United Nations workers in Karachi, the port city on the Arabian Sea, have also suffered attacks that have set back efforts to wipe the disease from the city, Pakistan's most populous.
Pakistan is one of three countries, along with Nigeria and Afghanistan, where polio remains endemic.
This article originally appeared in The New York Times.