Gunmen on motorcycles shot a polio worker and one of the police officers escorting him during a vaccination campaign in northwest Pakistan on Wednesday, police reported.
The attack took place in Bajaur, a district in the Khyber Pakhtunkhwa province near the Afghan border, a former stronghold of the Pakistani Taliban.
"After completing their duties, a polio team was returning to the local health unit when two unidentified motorcyclists opened fire on them," Waqas Rafiq, a senior police officer, told the AFP news agency. Rafiq said a third person was injured.
No one has yet claimed responsibility for the attack, according to local police chief Abdul Aziz.
Mohsin Naqvi, Pakistan's Interior Minister, condemned the attack.
Pakistan aims to vaccinate millions of children in seven days
On Monday, Pakistan launched a nationwide campaign to vaccinate more than 33 million children against polio within a week, after the first case was reported in the capital, Islamabad, in 16 years.
That same day, an improvised explosive device targeted a polio vaccination team in the same province, injuring nine people.
The "Islamic State" militant group claimed responsibility for the attack.
Anti-polio workers require escort in some regions
Police officers often escort polio workers who go door-to-door in conflict zones.
They are frequently attacked by militants who claim that vaccination campaigns are a Western conspiracy to sterilize children.
Disinformation, conspiracy theories and the influence of some radical Islamic clerics all contribute to this anti-vaccine narrative.
Hundreds of officials and polio workers have been killed over more than a decade.
What is polio?
Polio is a potentially deadly and paralyzing disease. It mainly affects children under 5 and is typically transmitted through contaminated water.
According to the United Nations Children's Fund (UNICEF), polio has drastically declined in Pakistan over the last three decades.
In the early 1990s, there were about 20,000 cases per year. By 2018, only eight were reported.
However, cases are rising again. According to Pakistan's Polio Eradication Program, 17 cases have been reported since January, 11 more than in the entire previous year.
Pakistan and Afghanistan are the only countries where polio remains endemic.