Four polio vaccination workers were shot and killed in three separate locations in Karachi, Pakistan, on Tuesday, police said. No one has claimed responsibility for the attacks, but the Taliban has issued threats against polio drive. The workers were reportedly part of a United Nations-backed program to eradicate polio, which has run rampant in Pakistan. In response to the attacks, Pakistani health officials halted the three-day nationwide anti-polio drive in Karachi. Pakistanis have been hostile to these immunization efforts in part due to the sham hepatitis C vaccination drive that helped lead the CIA to Osama bin Laden.