Karach: Backed by armed police guards, health workers on Tuesday began a three-day polio immunization drive in Karachi, Pakistan's most populous city. The health workers went from house to house to administer the polio drug, EFE news agency reported. Children up to the age of five were covered. Giving polio vaccine is fraught with danger in Pakistan, where the health workers are at times targeted by Islamist militants. One attack in January near a polio centre in Quetta city left 14 people dead. Some Islamic clerics and Taliban members say that the US-made polio vaccines can sterilise Pakistanis in a bid to reduce the country's Muslim population.