This month, militants in Pakistan tracked down and killed nine vaccination workers engaged in a three-day immunization campaign, the first time vaccinators have been targeted there in such large numbers. Although thousands of Pakistani health workers are pressing on with the effort, with the help of police protection, the United Nations has suspended its vaccination program in the country.

A health department worker administers a polio vaccine dose to a child in Peshawar, Pakistan, in October. EPA/Arshad Arbab

Polio kills about 200 Pakistanis a year, down from 1,500 in the mid-1990s, but the halting of the U.N. campaign means the country is even farther from eradicating the disease, as all but three other nations have managed to do. 

Beyond the humanitarian repercussions, meanwhile, the attacks on aid workers are also an example of why Pakistan has earned the dubious distinction of being ranked as one of top five least safe countries for aid workers, according to a new report from the group Humanitarian Outcomes.

After two years of decline, attacks on aid workers worldwide ticked up to 150 last year from 129 in 2010, and the number of victims of the attacks -- 308 -- was the highest ever recorded. But the vast majority of the attacks, 72 percent, occur in just five countries: Afghanistan, Somalia, Sudan, South Sudan and Pakistan. 

Screenshot: Humanitarian Outcomes

The causes of the risks to aid workers are what you'd expect: The attacks correlate strongly with political instability and conflict intensity, but overall high levels of domestic homicide do not have much to do with the aid worker murder rate:

The results indicated a moderately significant positive correlation between aid worker violence and presence of/intensity in fighting. There were also correlations between aid worker violence and (in descending order of significance) low levels of political stability, high ‘state fragility’ scores, institutional weakness of the regime, and low levels of ‘rule of law.’

The organization counts 12 incidents in Pakistan last year and 15 this year, but before 2009, there were only three or fewer attacks annually. Meanwhile, Sudan and Afghanistan have been consistently dangerous for aid workers throughout the past decade.

Pakistan's inclusion on the list this year is interesting because there is no intense conflict underway or brewing there. Instead, rule of law has been undermined by the increased influence of the Taliban and other militant groups since 2008.

Although wariness of vaccination programs first arose in Pakistan several years ago when radical clerics such as Maulana Fazlullah "warned" parishioners that polio vaccines cause infertility, more recently the Taliban has been responsible for much of the country's anti-vaccine sentiment.

The Islamist group denied responsibility for the recent aid worker attacks, but earlier this year, Taliban commanders in Pakistan’s northwestern tribal region said vaccinations could not proceed until the United States stopped drone strikes in the country. In July, the World Health Organization estimated that a polio vaccine ban could affect about 280,000 children living in tribal areas of northwestern Pakistan. 

The tensions over vaccines were exacerbated by the killing in Pakistan of Osama bin Laden, whose hideout was discovered in part through a CIA spy operation masquerading as a hepatitis vaccination program.

What's more, most of the vaccine workers who were gunned down were women, in line with Pakistani militants' apparent penchant for attacking female workers and advocates, including Malala Yousafzai, the school-age education reformer who was shot on a school bus in October for her widely publicized opinions on female education.

Unfortunately, the Pakistani government hasn't done much to improve the climate for health and other humanitarian workers. According to a recent Economist report, government officials there have restricted aid workers' movements and visas.

Armed escorts for aid operations in Pakistan became mandatory for international staff in some provinces after a string of kidnappings earlier this year, but according to Humanitarian Outcomes, such security measures can undermine the aid workers' independence. 

As one worker told the group: "It is presented as necessary for our own protection. Any incident will be bad PR for the government, and we’re at their invitation and they feel an obligation to protect us."