A woman holds her child as she walks past police-men outside a polio vaccination center in Karachi on January 8, 2013. PHOTO: AFP/ FILE
The victims of the shooting were aged 18 and 20. Police said both had died, but medical sources said one had died and one was seriously wounded.
No group claimed responsibility for the attack, though many extremists, including Taliban militants, have long opposed the campaign, convinced that the anti-polio drive is a Western plot to sterilise Muslims.
The United Nations in Pakistan pulled all staff involved in the campaign off the streets last December. Immunisation continued in some areas without UN support although many workers refused to go out.
"They were administering polio drops when they were attacked," police officer Siraj Ahmed said.
WHO spokesperson Maryam Younas said the organisation was temporarily suspending its polio vaccination drive in Peshawar.
"Operations will remain suspended in Peshawar until the security situation returns to normal," she said.
The latest violence has raised fresh questions over stability in the South Asian nation.
Critics say the attacks on the health workers are a prime example of the government's failure to formulate a decisive policy on tackling militancy, despite pressure from key ally the United States, the source of billions of dollars in aid.
Polio crippled thousands of people every year in rich nations until the 1950s. As a result of vaccination, it is now only endemic in three countries – Nigeria, Afghanistan and Pakistan.
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