The latest victim, a polio worker, was shot in southwestern Balochistan province Thursday morning, following separate attacks targeting health workers in the country's northwest earlier this week that left two police officers dead.
"The attack has taken place in a remote area, and we have received reports that one female polio worker has been killed," Attaullah Shah, a district police chief, told AFP, saying a second health worker was also critically injured in the attack.
Security officials in Pakistan told AFP that authorities have instructed polio teams to remain "vigilant" amid the violence.
The attacks come days after more than 25,000 children were rushed to hospitals in northwestern Pakistan after rumours spread some had suffered reactions to a polio vaccine.
The panic came as health workers were carrying out a three-day vaccination campaign in northwestern Khyber Pakhtunkhwa province, with authorities saying dozens of polio workers have been beaten or harassed.
Ongoing suspicion of the polio vaccine in Pakistan has been compounded recently by anti-vaccination videos circulating on social media.
Opposition to all forms of inoculation grew after the CIA organised a fake vaccination drive to help track down al Qaeda leader Osama Bin Laden in the Pakistani garrison town of Abbottabad. He was killed there by US forces in 2011.
"We have a communication challenge, it's a mistrust issue," said Babar Atta, who is helping oversee the country's vaccination drive.
Atta said at least 260,000 health workers are involved in the campaign with some 150,000 police escorting the teams.
However Atta added that hundreds of thousands of children were likely to go unvaccinated.
"There is a serious lack of trust among the parents," he said.
Polio is endemic in only three countries in the world - Pakistan, Afghanistan and Nigeria - although a relatively rare strain was also detected in Papua New Guinea at the end of last year.
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