The unidentified gunmen opened fire on health workers at Babar Kacha area in Dera Ismail Khan district of Khyber- Pakthunkhwa province.
The policemen were providing security cover to the vaccinators, sources said.
A policeman patrolling the area was killed instantly in the firing while another was critically wounded, they said.
Some officials, however, claimed the policemen were not attacked because of their deployment with polio teams. They said the policemen came under fire as security in Babar Kacha area is compromised.
Pakistan is one of only three countries where polio remains endemic. Efforts to eradicate it have been hampered by the targeting of vaccination teams in recent years.
The outlawed Tehrik-e-Taliban Pakistan has banned vaccination campaigns in parts of the tribal belt, putting tens of thousands of children at risk. Dozens of health workers and policemen have died in attacks on vaccination teams in the northwest and the port city of Karachi.