As health workers last week picked their way through the grimy lanes of one of Karachi's poorest slums, the virtual world of social media platforms would at first glance appear to pose little threat to their work.

Five years ago the women going door-to-door to distribute vaccines in a bid to eradicate polio defied the real physical threat of motorbike-riding militants who gunned down health workers across Pakistan.

That violence has thankfully receded, but the campaign to wipe out the crippling viral disease from one of its last haunts has this year instead faced a very 21st century challenge.

Each of the community workers knows when they knock on a door to vaccinate children inside, there is a chance parents will refuse because of a bogus video shared online which claims that the drops are deadly.

Last week a network of 250,000 workers accompanied by tens of thousands of police swung into action across Pakistan, as it does every month in a regular five-day vaccination drive.