– The long struggle to eliminate polio from Pakistan has faced many obstacles — public ignorance about the disease, myths about a Western plot to sterilize Muslims, claims about fake vaccination drives being used to cover up spying. Add to that list: Attacks by anti-government insurgents on teams delivering the polio vaccine, mostly around the lawless northwest tribal areas.

Now, just as the campaign was nearing its total eradication goal — with only eight new cases of the crippling virus reported in the past year — an especially coldblooded assault has left a mother and one of her seven children dead, slain on Thursday as they were distributing polio drops in the violence-plagued southwest city of Quetta.

The mother and her 16-year-old daughter were not accompanied by armed security guards, although they have long been provided for vaccination teams in Pakistan. A police official in Quetta said the local guards had been removed because they were "drawing attention." Pakistani commentators noted there is an ongoing debate about whether such guards help or hinder vaccinators' safety.

The anti-polio campaign is supported by the World Health Organization, UNICEF and other international agencies, which have assisted in successfully eradicating the virus in most of the world. The slain women were participating in a new drive in which hundreds of mobile vaccination teams are providing polio drops in cities across the country.

The killings were carried out by unknown drive-by killers, who shot the mother and daughter in the head as they sped past on motorbikes. There was no warning, no explanation, and no claim of responsibility. But the attack resembled previous ones by militants such as the Taliban, which have killed more than 100 Pakistani vaccinators and their police guards in the past five years.

Officials of national anti-polio groups vowed the latest attack would not deter or slow efforts to end polio in Pakistan, one of the last countries where the virus remains endemic. Another is ­Afghanistan.

"The killings are very, very sad, but they will not have any impact on the eradication campaign, because the federal government and all the international partners are fully committed to eradicate polio from Pakistan. We will not be brought down by this," said Aziz Memon, chairman of the Pakistan Polio Plus Committee in Karachi.

Memon said the last known attack on vaccinators in Balochistan Province, a vast desert region where Quetta is located, took place more than two years ago, when organized opposition to the anti-polio drive was high and many families were refusing to allow their children to be vaccinated.

Suspicion was aggravated in 2011 by the arrest of a Pakistani doctor who visited homes in a purported vaccination drive in the northwest region. He was later found to have been part of U.S. intelligence effort to locate Al-Qaida leader Osama bin Laden, who was then killed in a U.S. raid on a Pakistani compound in 2011.

Since then, some prominent Pakistani clerics have publicly endorsed the anti-polio campaign and attacks have become less frequent.

"We thought the refusal and violent agitation were over," Memon said. He suggested the Quetta attack could have been more broadly aimed at opposing the government, and he noted that three provincial policemen were also shot dead there the same day. The Pakistani Taliban claimed that attack.