Geo News said no one has claimed responsibility for the killing in Mardan, but last year, a Taliban faction banned polio vaccinations and claimed the health campaign was a cover for spying.
The campaign has also become unpopular due to rumors the vaccines sterilize Muslims, Geo News reported.
The killing Tuesday occurred on the second day of a three-day anti-polio campaign in the city, Geo News said, and the health workers were inside a house administering the vaccines when men on a motorbike drove by and shot the police guard standing outside.
The attack comes after gunmen killed two vaccination workers, a policeman and seven members of a charity in three attacks in January, Geo News said.
In December, gunmen killed nine members of a vaccination team.
Pakistan is just one of three countries where polio is still endemic, Geo News reported, and had the highest number of polio cases in the world in 2011.
Notable deaths of 2014 [PHOTOS]
More remains found at MH17 crash site in Ukraine