The aid workers were unhurt but five policemen were injured, the authorities said.
The attacks come amid a spike in violence committed by the local branch of the Taliban, Tehreek-e-Taliban-Pakistan, which operates along the rugged frontier between Pakistan and Afghanistan.
Recent attacks have hit army patrols and outposts, killing and injuring dozens of soldiers. Military operations in response have killed many of the militants.
The Taliban have a long history of attacking aid workers delivering polio vaccines. In the past, they have accused aid workers of taking part in a foreign conspiracy to sterilise the population.
Other vaccine drives have been accused of secretly working with foreign intelligence agencies to gather information on the group.
An incurable disease, polio mainly affects children under five years of age, and in some cases can cause paralysis and death, but it is preventable by an easily administered vaccine.
In this latest incident, police returned fire after coming under attack near a bridge in Dera Ismail Khan, a district in Khyber Pakhtunkhwa province, according to local police chief Aman Ullah.
The police chief, who was travelling with the convoy to a nearby vaccination site, said between six to eight suspected militants ambushed them, opened fire and threw hand grenades at the police van.
The latest anti-polio campaign started this week, the first in 2023.
In late November, a Taliban suicide bomber blew himself up near a lorry carrying police officers on their way to protect polio workers in the south-western city of Quetta. A police officer in the vehicle was killed, as were three members of a family travelling in another car.
The Pakistani Taliban claimed responsibility for the November 30 attack. A month earlier, in October, gunmen riding on a motorcycle shot and killed a police officer assigned to guard a polio vaccination team in south-western Balochistan province.
Pakistan, which along with neighbouring Afghanistan is the only country where polio remains endemic, regularly launches vaccination campaigns.
Since last April, Pakistan has registered 20 new polio cases — all in the north-west, where parents often refuse to inoculate children. The outbreak has been a blow to government efforts aimed at eradicating the disease.