Following the violence, the United Nations in Pakistan has pulled all staff involved in the immunisation campaign off the streets, spokesman Michael Coleman said.
A senior World Health Organization (WHO) official said that 2,200 field workers had been directed to stop their operations.
It is not clear exactly who is behind the violence but some Islamists, including Taliban militants, have long opposed the campaign, with some saying it is aimed at sterilising Muslims.
Yesterday saw at least three separate attacks. In the northwestern district of Charsadda, men on motorbikes shot dead a woman and her driver, police and health officials said.
Hours earlier, a male health worker was shot and badly wounded in the nearby provincial capital of Peshawar. He remains in a critical condition, said a doctor at the hospital where he is being treated.
Four other women health workers were shot at but not hit in nearby Nowshera, said Jan Baz Afridi, deputy head of WHO’s Expanded Programme on Immunisation.
The Taliban have repeatedly issued threats against the polio eradication campaign and health workers said they received calls telling them to stop working with the ‘infidels.’
A spokesman for the Pakistani Taliban, Ihsanullah Ihsan, said his group was not involved in the violence.