Update to this story.

(Reuters) — Three workers in a polio eradication campaign were shot Wednesday in Pakistan, and two of them were killed, the latest in an unprecedented string of attacks over the past three days that has partially halted the U.N.-backed campaign.

The United Nations in Pakistan has pulled all staff involved in the immunization campaign off the streets, spokesman Michael Coleman said.

The government said immunization was continuing in some areas without U.N. support although many workers refused to go out. Women health workers held protests in the southern city of Karachi and in the capital, Islamabad.

“We go out and risk our lives to save other people’s children from being permanently handicapped, for what? So that our own children become orphans?” health worker Ambreen Bibi said at the Islamabad protest.

The government was caught off guard by the violence, saying they had not expected attacks in areas far from Taliban strongholds and they would have to change tactics in the health campaign.

Wednesday saw four separate attacks, all in the north. In the district of Charsadda, men on motorbikes shot dead a woman and her driver, police and health officials said.