"Two gunmen hiding near a small water channel opened fire on the policemen from very close range," senior police officer Waqar Ahmad Khan told AFP on Tuesday.
Vaccination teams in Pakistan and neighbouring Afghanistan have for years been targeted by militants.
The two countries are the only ones where polio is endemic.
"The gunmen spared the two-member polio vaccination team ... and fled on a motorbike," Mr Khan said.
The incident occurred in Kot Azam, Tank district, close to tribal districts where the military has clashed with militants since 2003.
Scores of polio workers and security officials guarding them have been killed since 2012 by militants who believe vaccination programmes are part of a western plot to sterilise Muslims.
Opposition by militants to inoculation campaigns grew after the CIA organised a fake vaccination drive to help track down Al Qaeda's former leader Osama bin Laden in the Pakistani city of Abbottabad.
In April, Pakistan reported the first case of polio in 15 months.
Since then, 14 more polio cases have been reported, all from the same ultra-conservative district where many villagers are against vaccines.
The US reported its first case of polio in almost a decade in July. The UK said last week that about a million children in London would be offered a booster vaccine after the virus was detected in sewage samples.