Health teams in Pakistan have been attacked repeatedly since the Taliban denounced vaccines as a Western plot to sterilize Muslims and imposed bans on inoculation in June 2012.

In North Waziristan, a region near the Afghan border that has been cordoned off by the Taliban, dozens of children, many under the age of two, have been crippled by the viral disease in the past six months.

And there is evidence in tests conducted on sewage samples in some of the country's major cities that the polio virus is starting to spread beyond these isolated pockets and could soon spark fresh polio outbreaks in more densely populated areas.