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.